Decline in coital rates with spouses' ages and duration of marriage

Foetal loss and feminine sex ratios at birth in sub-Saharan Africa

A wealth of demographic research has explored the determinants of sex ratios at birth, but few studies have considered the role of foetal loss (spontaneous abortion), in producing feminine sex ratios. One challenge is measuring the occurrence of foetal loss, which is difficult to recognize and report in survey research. This study uses the length of the birth interval as a proxy for foetal loss; foetal loss restarts the clock on time to conception and lengthens the birth interval. We use Demographic and Health Survey data on second births to women in 17 sub-Saharan African countries. Results show that longer second birth intervals are significantly related to lower odds of a male second birth and to feminine sex ratios at birth. These findings suggest that high levels of foetal loss, which could signal underlying poor maternal health in a population, have dramatic effects on the sex ratio at birth.

Influence of the Super Bowl on the United States birth sex ratio

INTRODUCTION

Many factors have been shown to influence the male to female ratio at birth (M/T). Celebrations increase M/T nine months later, putatively due to elevated coital rates. The Super Bowl (SBS) is consistently held in the first Sunday in February. This study was carried out in order to ascertain whether SBS results in a rise in M/T nine months later, and whether any such changes are influenced by race and census region.

METHODS

Births were studied for the period 2003-2015. Seasonality adjustment was applied in order to identify true outlier values over and above those due to natural seasonal M/T variation.

RESULTS

This study analysed 53,105,069 births. M/T was elevated in all regions, for all races for 11/2006, 9/2009, 10/2009, 10/2010, 12/2010, 11/2011, 11/2013 and 12/2013. A similar effect is observed in the West Region for all births. For the various ethnic groups, the effect is strongest for Whites in regions Northeast, South and West, and for Black/African American births in regions Midwest and South.

DISCUSSION

Events which increase coital activity elevate M/T nine months later as M/T follows a U-shaped regression on cycle day of insemination, with male conceptions occurring more frequently at the beginning (and end) of the menstrual cycle. SBS is a significant annual celebratory event in contemporary American culture. It is possible that increased coital rates during this period (perhaps due to a combination of celebration and inebriation) may result in the M/T rises nine months later observed in this study.

Offspring sex ratio: Coital rates and other potential causal mechanisms

In recent years, scientists have begun to pay serious attention to the hypothesis that human parental coital rates around the time of conception causally influences the sexes of subsequent births. In this paper, the grounds of the argument are outlined. The point is important because, if the hypothesis were credible, it can potentially explain one of the best established (and otherwise unexplained) epidemiological features of sex ratio at birth - its rises during and just after World Wars 1 and 2 insofar as increased coital rates increase the ratio. Moreover, the greater the understanding of the variations of sex ratio at birth, the greater will be the understanding of the causes of those selected diseases associated with unusual sex ratios at birth (testicular cancer, hepatitis B, Toxoplasma gondii, and, perhaps, prostatic cancer).

Further evidence of male offspring preference for certain subgroups in the United States (2007-2015)

INTRODUCTION

Males are born in excess of females. This ratio (M/T=male/total births) is influenced by various factors. A recent study showed that M/T in the United States (US) is Asian or Pacific Islander>White>American Indian or Alaska Native>Black or African American. This study was carried out in order to ascertain whether there are M/T differences in different races in the US by birth order.

METHODS

Monthly male and female live births by race and birth order for the entire US were obtained for 2007-2015 with birth order as 1-≥6 for these races.

RESULTS

There were 36,499,163 births. M/T decreased with increasing sibling order for all races (p<0.0001) except for Asian or Pacific Islander births where M/T rose progressively to 3rd order births (p<0.0001) then fell (p=0.0002). Weighted mean maternal age for each birth order by race order was Asian or Pacific Islander>White Black or African American/American Indian or Alaska Native. The differences between adjacent means were all significant (almost all p<0.0001). The mean maternal age spread followed the same order.

DISCUSSION

Asians favour male offspring. This group may be systematically implementing foetal sex-specific feticide and/or implementing a Type1 stopping rule when a male birth is achieved in order to maximise male births. The potential putative effect of increasing maternal age to decrease M/T is not only excluded in this race, but goes contrary to the findings of this study insofar as Asian or Pacific Islander births have the oldest mean maternal ages. Son preference strongly persists in the US.

Testosterone and libido in surgically and naturally menopausal women

The assessment and then treatment of a change in libido, or a change in the desire to partake in sexual activity, during the menopausal transition and beyond has been a challenging and elusive area of clinical research. This is partly due to the multidimensional nature of female sexuality, the difficulties of measuring testosterone in women in a reliable and accurate manner, and the complexity of the neurobiology and neurobehavior of female sexual desire. In addition, there is a lack of evidence for diagnostic specificity of low free testosterone levels for the symptom of low libido in women for whom there are no confounding interpersonal or psychological factors; although, in the symptomatic population of surgically or naturally menopausal women, a low level of free testosterone often accompanies a complaint of reduced desire/libido. The randomized clinical trial research on testosterone replacement for naturally and/or surgically menopausal women with sexual dysfunction has been criticized for a high placebo response rate, supraphysiological replacement levels of testosterone, the perception of modest clinical outcome when measuring objective data such as the frequency of sexual intercourse relative to placebo, and the unknown safety of long-term testosterone replacement in the estrogen-replete surgically or naturally menopausal woman. A careful review of current evidence from randomized, controlled trials lends support to the value of the replacement of testosterone in the estrogen-replete menopausal woman for whom libido and desire has declined. The issue of long-term safety remains to be answered.

Prevalence, risk factors, and predictors of female sexual dysfunction in a primary care setting: a survey finding

INTRODUCTION

Female sexual dysfunction (FSD) is a highly prevalent sexual health problem but poorly investigated at the primary care level.

AIM

This article examines the prevalence of sexual dysfunction and its possible risk factors associated with women at high risk of FSD in a hospital-based primary practice.

METHODS

A validated Malay version of the Female Sexual Function Index (MVFSFI) was utilized to determine FSD in a cross-sectional study design, involving 163 married women, aged 18-65 years, in a tertiary hospital-based primary care clinic in Kuala Lumpur, Malaysia. Sociodemographic, marital profile, health, and lifestyle for women at high risk of FSD and those who were not at high risk were compared and their risk factors were determined.

MAIN OUTCOME MEASURES

Prevalence of FSD in Malaysian women based on the MVFSFI, and its risk factors for developing FSD.

RESULTS

Some 42 (25.8%) out of 163 women had sexual dysfunction. Prevalence of sexual dysfunction increased significantly with age. Sexual dysfunctions were detected as desire problem (39.3%), arousal problem (25.8%), lubrication problem (21.5%), orgasm problem (16.6%), satisfaction problem (21.5%) and pain problems (16.6%). Women at high risk of FSD were significantly associated with age (OR 4.1, 95% CI 1.9 to 9.0), husband's age (OR 4.3 95% C.I 1.9 to 9.3), duration of marriage (OR 3.3, 95% CI 1.6 to 6.8), medical problems (OR 8.5, 95% CI 3.3 to 21.7), menopausal status (OR 6.6, 95% CI 3.1 to 14.3), and frequency of sexual intercourse (OR 10.7, 95% CI 3.6 to 31.7). Multivariate analysis showed that medical problem (adjusted OR 4.6, 95% CI 1.6 to 14.0) and frequency of sexual intercourse (adjusted OR 7.2, 95% CI 2.1 to 24.0) were associated with increased risk of having FSD. Those who practiced contraception were less likely to have FSD.

CONCLUSION

Sexual health problems are prevalent in women attending primary care clinic where one in four women were at high risk of FSD. Thus, primary care physician should be trained and prepared to address this issue.

Correlates of sexual functioning among mid-life women

OBJECTIVE

Studies have reported a decline in sexual functioning among women undergoing the menopausal transition. Few studies, however, have examined the associations between hormones and sexual dysfunction during this time period. Therefore, the purpose of this study was to examine the associations between participant characteristics and endogenous hormones with sexual functioning in mid-life women.

METHODS

Data were analyzed from a community-based sample of 441 women aged 45-54 years who stated that they were sexually active at the time of the study. Each participant completed a survey that included questions pertaining to sexual functioning and provided a blood sample that was used to measure estrogen and androgen concentrations.

RESULTS

Among women who reported being sexually active, poorer self-reported health and the experiencing of depressive symptoms were significantly associated with not being satisfied with sexual relations after adjustment for other covariates. None of the hormones examined were significantly associated with overall sexual satisfaction. However, statistically significant associations between both total testosterone levels and the free testosterone index with satisfaction with the frequency of sexual relations were observed.

CONCLUSIONS

Our findings indicate that the experiencing of depressive symptoms and the reporting of poor overall health are important correlates of sexual dysfunction. Further, our results suggest that higher total and free testosterone levels are significantly associated with a desire for increased frequency of sexual relations among mid-life women.

Sexuality and the menopause

Sexuality is innate within all women to a greater or lesser extent, and is affected by a number of extrinsic factors that occur in the menopausal transition. Assessing hormone status is difficult as evidence exists that sex hormones may differ between ethnic groups, and that bio-assays may be insensitive at lower testosterone levels. Data are available on the prevalence of female sexual dysfunction, but results from cross-sectional studies differ from those of longitudinal studies. The original traditional models of human sexual response have been challenged, and new models have been defined which show more complex interaction between intrinsic and extrinsic factors. Definitions of sexual dysfunction have been redefined. There are a limited number of randomized, placebo-controlled trials of drugs to improve sexual function. These include sildenafil citrate, tibolone and hormone replacement therapy. Randomized controlled trials on testosterone replacement in naturally and/or surgically menopausal patients with female sexual dysfunction have been criticized for a high placebo response rate and short duration. This chapter seeks to put sexuality into perspective and to define both function and dysfunction.

Factors affecting sexuality in older Australian women: sexual interest, sexual arousal, relationships and sexual distress in older Australian women

OBJECTIVE

To investigate the sexual behavior, sexual relationships, sexual satisfaction, sexual dysfunction and sexual distress in a population of older urban Australian women.

METHOD

In 2004, 474 women participating in the Longitudinal Assessment of Ageing in Women (LAW) Study completed a series of questionnaires about sexuality. They included the Short Personal Experiences Questionnaire (SPEQ), Relationship Assessment Scale (RAS), Female Sexual Distress Scale (FSDS), questions concerning past sexual abuse based on the Sex in Australia Study, and questions comparing present and past sexual interest and activity.

RESULTS

The percentage of women with partners ranged from 83.3% in the 40 - 49-year age group to 46.4% women in the 70 - 79-year age group. The sexual ability of partners diminished markedly with age, with only 4.8% of the partners using medication to enable erections. Only 2.5% of women reported low relationship satisfaction. The incidence of sexual distress was also low, being reported by only 5.7% of women. Younger women and women with partners had higher levels of distress than older women. Indifference to sexual frequency rose from 26.7% in women aged 40 - 49 years to 72.3% in the 70 - 79-year age group. Past sexual abuse was recalled by 22.7% of women and 11.6% recalled multiple episodes of abuse. Women who recalled abuse had lower scores for satisfaction with sexual frequency.

CONCLUSIONS

It appears from this study that there is a wide range of sexual experience amongst aging women, from never having had a sexual partner, to having solitary sex, to having a relationship with or without sex into the seventh decade. As women age, they experience a decrease in sexual activity, interest in sex, and distress about sex. This may be associated with the loss of intimate relationships as part of separation, divorce or bereavement. Decreased sexual activity with aging may be interpreted as a biological phenomenon (part of the aging process) or as sexual dysfunction, or it may be the result of adapting to changed circumstances.

Factors associated with determinants of sexual functioning in midlife women

OBJECTIVES

The present study was conducted to assess the associations between a woman's passionate love for her partner and sexual satisfaction and demographic factors, health and life-style characteristics, menopausal status, and menopausal symptoms.

METHODS

A cross-sectional study was conducted among women (40-60 years) residing in Maryland (n = 846). chi(2)-tests were performed to evaluate the associations between each of the outcome variables (sexual satisfaction and passionate love for the partner) and predictor/independent variables. Logistic regression analysis was performed to determine whether significant associations from chi(2) analyses remained significant after adjustment for confounders.

RESULTS

Older age (adjusted odds ratio (OR) 1.04; 95% confidence interval (CI) 1.01, 1.07), higher education (OR 1.47; 95% CI 1.09, 1.99), and alcohol intake (OR 1.42; 95% CI 1.03, 1.95) were associated with low passion for the partner. Older age was also a significant predictor of low sexual satisfaction (OR 1.04; 95% CI 1.01, 1.07). Women experiencing vaginal dryness had higher odds of low passion for partner (OR 1.67; 95% CI 1.21, 1.31) and low sexual satisfaction (OR 1.58; 95% CI 1.14, 2.20) than women not experiencing vaginal dryness.

CONCLUSIONS

Older age, higher education, alcohol intake, and vaginal dryness are significantly associated with lower levels of passionate love for the partner, while older age and vaginal dryness are significantly associated with lower levels of sexual satisfaction in midlife women.

Sexuality

This article reviews changes in sexual function in middle-aged women and discusses how these changes relate to aging, hormone alterations, and psychosocial and physical factors. A Medline search of population-based studies that measured sexual function, menopausal status and/or hormone levels was conducted. Longitudinal findings are from the Melbourne Women's Midlife Health Project, a population-based sample of 438 Australian-born white women, aged 45 to 55 years, who were menstruating at baseline. Annual assessments included hormone levels and the Short Personal Experiences Questionnaire. Few of the population-based studies of the menopausal transition measured sexual function or hormones. Aging and the length of the woman's relationship with her partner are associated with decline in sexual function. An additional decrement in sexual function occurs in midlife associated with menopause. Findings from the Melbourne Women's Midlife Health Project using structural equation modeling, found the most important factors influencing a woman's sexual function are prior level of sexual function; losing or gaining a sexual partner; feelings toward a partner; and estradiol level. When psychosocial and lifestyle status were added to the model, mood was the only additional variable affecting sexual function. There is a decline in all aspects of female sexual function with age. A further incremental decline in most aspects of sexual function occurs as women pass through the menopausal transition and is related to decreasing estradiol levels. Other factors such as prior sexual function and partner issues have larger effects on women's sexual function than do hormonal factors.

The relative effects of hormones and relationship factors on sexual function of women through the natural menopausal transition

OBJECTIVE

To investigate the relative effects of hormonal and relationship factors on female sexual function during the natural menopausal transition.

DESIGN

Prospective population-based questionnaire study.

SETTING

Interviews were conducted in the patients' homes.

PATIENT(S)

Four hundred thirty-eight Australian-born women aged 45-55 years who were still menstruating at baseline. Eight years of longitudinal data were available for 336 of these women, none of whom were hysterectomized.

INTERVENTION(S)

Hormonal levels, age, menopausal status, partner status, and feelings for partner were measured and evaluated with longitudinal structural equation modeling.

MAIN OUTCOME MEASURE(S)

Short personal experiences questionnaire.

RESULT(S)

Sexual response was predicted by prior level of sexual function, change in partner status, feelings for partner, and E2 level (R2 = .65); dyspareunia was predicted by prior level of dyspareunia and E2 level (R2 = .53); and frequency of sexual activities was predicted by prior level of sexual function, change in partner status, feelings for partner, and level of sexual response (R2 = .52). The minimum effective dose needed to increase sexual response by 10% (700 pmol/L E2) is twice that needed to decrease dyspareunia.

CONCLUSION(S)

Prior function and relationship factors are more important than hormonal determinants of sexual function of women in midlife.

The Impact of Aging on Sexual Function and Sexual Dysfunction in Women: A Review of Population‐Based Studies

INTRODUCTION

Scientific interest in the impact of aging on women's sexual function and dysfunction has increased in the half century since Kinsey described age-related changes in women's sexual activities. However, a range of methodological issues limit the conclusions that can be drawn from many published studies in this area.

AIM

To review community-based studies investigating changes in women's sexual function and sexual dysfunction with age, taking into account confounders to aging and methodological limitations.

METHODS

Electronic databases were searched for published studies investigating changes in sexual function and dysfunction with age. A critical review was carried out.

MAIN OUTCOME MEASURES

Age-related changes in sexual function and dysfunction.

RESULTS

There are inconsistencies in the way sexual function and sexual dysfunction are measured. Validated scales are infrequently used. Low response rates, limited age ranges, and restrictive inclusion criteria limit the generalizability of many studies. Confounders are often either not measured or not analyzed. Longitudinal studies are rare, making it difficult to separate the effects of birth cohort and aging. The evidence indicates that a woman's sexual function declines with age. This decline begins in a woman's late 20s to late 30s. Specifically, desire, frequency of orgasm, and frequency of sexual intercourse decrease with age. However, it is not clear whether arousal decreases or remains relatively constant. In longitudinal studies, decline in women's sexual function has also been detected, but patterns of stability and improved sexual function have also been observed for short periods of time. The prevalence of most sexual difficulties or dysfunctions changes little with age, with the exception of sexual pain, which may decrease.

CONCLUSIONS

Age-related changes in sexually related personal distress may help explain why the prevalence of sexual dysfunctions remains constant with age while sexual function declines. More research is needed to demonstrate this.

Perimenopausal androgen decline after oophorectomy does not influence sexuality or psychological well-being

OBJECTIVE

To determine whether oophorectomy during the perimenopause, with the associated decline in ovarian androgens, affects sexual function and psychological well-being negatively.

DESIGN

Prospective, observational study comparing sexuality and psychological well-being in women after hysterectomy only (HYST) vs. hysterectomy and concomitant oophorectomy (HYST+BSO).

SETTING

University hospital and district general hospital.

PATIENT(S)

Three hundred sixty-two perimenopausal women scheduled for elective hysterectomy on benign indication were recruited and 323 (89%) completed the 1-year follow-up (217 in the HYST group and 106 in the HYST+BSO group).

INTERVENTION(S)

The patients were evaluated preoperatively and 1 year after surgery. Postoperatively, estrogen replacement therapy was recommended to all women in the HYST+BSO group and to HYST group subjects with climacteric symptoms.

MAIN OUTCOME MEASURE(S)

Sex steroids (T, androstenedione, DHEA-S, and E(2)) and sex hormone-binding globulin (SHBG) were measured. Free androgen index and free E(2) index were calculated. Sexuality (McCoy's Female Sex Questionnaire) and psychological well-being (Psychological General Well-Being Index) were evaluated.

RESULTS(S)

Preoperatively, no hormonal differences were found between the two groups. At 1-year follow-up, all sex steroid levels and indices were decreased and SHBG was increased in the HYST+BSO group. Ovarian sex steroids were decreased in the HYST group, whereas DHEA-S and SHBG were unaltered. Sexuality was unaltered in the HYST+BSO group, whereas decreased scores were found in 3 of 14 sexual variables in the HYST group. Psychological well-being was improved in both groups. There were no correlations between the observed changes (data 1 year after surgery, compared with preoperative data) in androgen levels and index and the observed changes in any aspect of sexuality or psychological well-being.

CONCLUSION(S)

Hormonal changes after oophorectomy in conjunction with perimenopausal hysterectomy do not significantly change postoperative (1-year) sexual or psychological well-being.

The age pattern of fecundability: an analysis of French Canadian and Hutterite birth histories

This paper analyzes the age pattern of effective fecundability from populations with no evidence of deliberate fertility control using a new convolution model of fecundability. The analysis is based on a sample of Hutterite birth histories from the mid-20th century, and birth histories of French Canadians from the 17th and 18th centuries. The main findings are as follows: 1) the level of effective fecundability is higher among the French Canadians compared to the Hutterites; 2) effective fecundability peaks at age 20 for the Hutterites, and in the early to mid-20s for the French Canadians; 3) Hutterite effective fecundability declines almost linearly from age 20 to 45, and French Canadian effective fecundability declines slowly from its peak to the early 30s, and more rapidly at older ages; and 4) the duration of postpartum amenorrhea is longer for the French Canadians than for the Hutterites. Because of the shorter periods of postpartum amenorrhea the Hutterites have about the same average number of children as the French Canadians, even though the French Canadians have higher effective fecundability.

Are changes in sexual functioning during midlife due to aging or menopause?

OBJECTIVE

To determine whether changes in women's sexual functioning during midlife are due to aging or menopause.

DESIGN

Prospective, observational study.

SETTING

Population-based sample assessed in own homes.

PATIENT(S)

Four hundred thirty-eight Australian-born women aged 45-55 years and still menstruating at baseline. One hundred ninety-seven were studied for effects of the natural menopausal transition. Control group A (n = 44) remained premenopausal or early perimenopausal for 7 years. Control group B (n = 42) remained postmenopausal over 5 years.

INTERVENTION(S)

Nil; questionnaires and blood sampling annually.

MAIN OUTCOME MEASURE(S)

Shortened version of the Personal Experiences Questionnaire.

RESULT(S)

By the late perimenopause, there was a significant decline in the factors we had derived of sexual responsivity and total score, and there was an increase in the partner's problems factor. By the postmenopausal phase, there was a further decline in the factors sexual responsivity, frequency of sexual activities, libido, and in the total score, and a significant increase in vaginal dyspareunia and partner's problems. Sexual responsivity significantly declined in both control groups.

CONCLUSION(S)

Sexual responsivity is adversely affected by both aging and the menopausal transition. Other domains of female sexual functioning were significantly adversely affected when the women became postmenopausal. The relationship with the partner and his ability to perform sexually is adversely affected by the menopausal transition.

Factors affecting sexual functioning of women in the mid-life years

OBJECTIVE

To model the interaction of hormones, symptoms and psychosocial factors on women's sexuality during the menopausal transition.

DESIGN AND METHODS

A prospective, observational study was carried out of a community-based sample of 438 Australian-born women aged 45-55 years at baseline. The study comprised six annual assessments in the women's own homes utilizing a core questionnaire, with rating scales for well-being and daily hassles, and a Personal Experiences Questionnaire as a measure of sexual functioning. Levels of follicle stimulating hormone (FSH), estradiol and inhibin were measured annually. Statistical analysis was performed by structural equation modelling.

RESULTS

The retention rate was 90% (final sample size after exclusions, n = 354). The normal fit index for the global model obtained was 0.92. There is a significant direct effect of menopausal status on vaginal dryness/dyspareunia, and an indirect effect on sexual responsivity via a direct effect of menopausal status on symptoms, which then affect well-being. Menopausal status reflects hormonal status. Feelings for the partner and the partner's sexual problems have direct effects on different aspects of sexual functioning. Other social variables such as paid work, interpersonal stress, daily hassles and educational level affect sexual functioning indirectly via effects on symptoms and well-being.

CONCLUSIONS

Psychosocial factors, symptoms and the menopausal transition affect women's sexual functioning during the mid-life years.

Predictors of fecundability and conception waits among the Dogon of Mali

Surprisingly little is known about the mechanisms that underlie variation in female fertility in humans. Data on this topic are nonetheless vital to a number of pragmatic and theoretical enterprises, including population planning, infertility treatment and prevention, and evolutionary ecology. Here we study female fertility by focusing on one component of the interbirth interval: the waiting time to conception during menstrual cycling. Our study population is a Dogon village of 460 people in Mali, West Africa. This population is pronatalist and noncontracepting. In accordance with animist beliefs, the women spend five nights sleeping at a menstrual hut during menses. By censusing the women present at the menstrual huts in the study village on each of 736 consecutive nights, we were able to monitor women's conception waits prospectively. Hormonal profiles confirm the accuracy of the data on conception waits obtained from the menstrual hut census (Strassmann [1996], Behavioral Ecology 7:304-315). Using survival analysis, we identified significant predictors of the waiting time to conception: wife's age (years), husband's age (< 35, 35-49, > 49 years), marital duration (years), gravidity (number of prior pregnancies), and breast-feeding status. Additional variables were not significant, including duration of postpartum amenorrhea, sex of the last child, nutritional status, economic status, polygyny, and marital status (fiancée vs. married). We fit both continuous and discrete time survival models, but the former appeared to be a better choice for these data.

Sex ratio, coital rate, hormones and time of fertilization within the cycle

In this paper I shall consider the relationships between coital rates, timing of fertilization within the cycle, and sex ratios of the resulting offspring. It will be suggested that there is: (1) Analytic (mathematical) evidence for a correlation between coital rate and time of fertilization. (2) Empirical evidence for a correlation between time of fertilization and sex ratio of offspring. Whence there arises: (3) A correlation between coital rate and sex ratio secondary to correlations of both with time of fertilization. These variables will be considered in these pairs in order. Section 4 will be devoted to the magnitude of the correlation between coital rate and sex ratio. Lastly it will be noted that the data on these two variables cohere not only with that on timing of insemination, but with the overall hypothesis that parental hormone levels at conception are causally associated with sex ratios at birth.

Hormonal and behavioral determinants of the secondary sex ratio

The timing of insemination relative to ovulation and the frequency of insemination appear prominently in analyses of variations in human secondary sex ratios. Explanations invoking these variables are shown to be inadequate. A new synthetic model of sex determination is proposed in which the sex of offspring is powerfully determined by the state of the cervical mucus. The cervical state is then shown to be a function of hormonal factors endogenous to the female in interaction with the effects of previous inseminations.

What stabilizes the sex ratio?

It is suggested that the human sex ratio at birth is stabilized only to a minor extent by the direct processes of natural selection. Instead the major factors stabilizing sex ratio seem to be behavioural (coital rates) and psychological (parental perceptions of adult sex ratios). It is suggested that parental hormone levels are (a) a consequence of perceived adult sex ratios, and (b) a cause of sex ratio in the next generation, thus providing the basis for a negative feedback process stabilizing the sex ratio.

The end of fertility: age, fecundity and fecundability in women

Onset of capacity for childbearing in women is dated biologically by menarche, although actual onset may be delayed. The end of childbearing is less understood but recent demographic and biological research on fertility at older ages in clarifying the end of fertility. The demographic view of declining fertility with age is based on age-specific fertility in natural fertility populations, artificial insemination and pregnancy rates by age and World Fertility Survey data. New data from the Demographic and Health Surveys on exposure to the risk of pregnancy shows that whereas older women biologically need longer exposure to pregnancy, exposure declines on behavioural grounds such as duration of marriage. Actual fecundity is obscured by factors of fecundability. Recent research on medically assisted conception is adding to the understanding of declining fecundity with age, especially the relative contributions of endometrial and ovarian ageing. This paper reviews the available information on declining fertility with age and discusses the implications of the extension of fertility through new medical technologies.

Modeling Seasonality in Fecundability, Conceptions, and Births

This paper develops a model of seasonal fluctuations in fecundability, conceptions, and births. We begin with a model of individual fecundability that combines behavioral and biological components, with particular attention to the roles of coital frequency, sperm concentration, fetal loss, and contraception. The individual-level model is then expanded into a model of seasonal fluctuations in births at the population level, which accounts explicitly for seasonal fluctuations in the size of the susceptible population. We illustrate the use of the model by analyzing proposed explanations of birth seasonality that rely on extreme summer heat.

Implications of hormonal influences on sexual behavior for demographic models of reproduction

In exploring the implications of hormonal influences on sexual behavior for reproduction, we have focused on androgens because of the convincing evidence for androgenic effects on female sexual motivation. We have been guided by the simple idea, based on clinical findings among hyperandrogenic women, that higher testosterone levels will increase female sexual motivation which in turn will result in increased coital frequency. However, careful consideration of the evidence fails to confirm such a role for testosterone in sexual behavior among normal women at any point in the reproductive span. While some amount of testosterone appears to be important in maintaining female sexual motivation, there is little evidence that variation in testosterone within the normal range is associated with variation in sexual motivation. Reported associations between testosterone and sexual behavior among married women cannot be interpreted as resulting from androgenic effects on sexual motivation. Thus we are left with the task of explaining why testosterone does not appear to play the same role in libido among hyperandrogenic and normal women. As mentioned earlier, some of the difference may attributable to the much higher levels of testosterone among hyperandrogenic women. Sherwin points out that the levels of sexual motivation decline with declining testosterone levels even while testosterone is well above normally occurring levels. It is possible that the brain is simply not sensitive to the variation in testosterone levels found in normal women. Other evidence suggests that the presence of intact ovaries may be equally important. A recent study of androgen replacement in naturally post-menopausal women failed to find a dramatic effect of testosterone on sexual motivation and behavior, despite levels similar to those in studies on surgically menopausal women, pointing to the importance of other factors associated with the presence of ovaries. A similar point can be made with regard to hyperandrogenism related to endogenous sources of testosterone, as in the case of PCOS. High levels of testosterone effectively disrupt ovarian function and interfere with other ovarian processes. Among hyperandrogenic women sexual behavior appears to be related to the direct effects of androgens on motivation, while the indirect effects of estrogen and progesterone are essentially eliminated. Among normal women, on the other hand, there is little evidence for such a dominating role of a direct androgenic effect on sexual behavior. Instead, other ovarian hormones, including estrogen and progesterone may also play a demonstrable role, despite the lack of strong evidence at this point.(ABSTRACT TRUNCATED AT 400 WORDS)

Behavioral and biological determinants of fecundability

Our discussion has focused on a set of behavioral and physiological factors which influence fecundability. Some potentially important determinants are notable primarily because of their absence. Foremost among these determinants (as a group) are the male attributes. We have, by and large, ignored the links between male sexual behavior and hormonal profiles and the effects of hormones (or other factors) on sperm function. The fact that we have not reviewed them here does not imply that we believe that they are unimportant. Indeed, the effects of male factors may be particularly important because in some settings men are the predominant, if not the sole, decision-makers regarding reproductive behaviors including initiation of sexual intercourse and use of contraception. Also conspicuous by their absence are the effects of nutrition, consumption of alcohol, exercise, smoking, and behavioral and hormonal correlates of stress, all of which may affect fecundability. We have, though, reviewed a wide range of factors which contribute to fecundability and identified several points of potential interaction between the behavioral and biological determinants. We expect that our future understanding of these processes will be enhanced by empirical work and modeling efforts which also interact to draw upon the insights provided by each.

Secondary sex ratios in Micronesian populations

Secondary sex ratio (SSR) values in excess of 109 male for every 100 female livebirths have been reported from several Micronesian populations. Analyses of linked family records dating from before 1942 for the native people of Guam reveal a highly masculinized SSR even at high birth orders and parental ages of married fertile couples. These findings contrast with reported declines in SSR with parity and parental age among historical and modern populations in Europe and North America where such declines often have been ascribed to the effects of decreasing mean coital frequencies associated with marital duration and spousal age. This study challenges implicit assumptions about the supposed universality of Euroamerican patterns of marital sexual behavior and supports the coital frequency hypothesis of SSR determination.

How does variation in fetal loss affect the distribution of waiting times to conception?

Unmeasured variation has long been a concern in analyses of the waiting time to conception. Recent work by Heckman and Walker (1991) and Trussell and Rodriguez (1990) has underscored the fact that statistical considerations alone cannot discriminate among likely models describing the distribution. Here, we specify a single theoretically important source of heterogeneity, namely variability in intrauterine mortality, and assess its effects on the waiting times to a conception which results in a live birth. We find that the effects on the waiting times to a conception which results in a live birth. We find that the effects of variation in fetal loss are confined to the tail of the distribution. Unless variation in fetal loss is extremely large or a substantial proportion of observed waiting times are initiated at late ages, variation in fetal loss does not appear to explain much variation in conception waits. We conclude that heterogeneity in fetal loss does not explain the variation in fecundability that has been observed for first birth intervals. This conclusion supports the hypothesis that at early ages (below age 35) variation in the waiting time to a fertile conception may largely reflect the proportion of nonsusceptible couples in the population. The analyses suggest that for the purposes of testing theoretically motivated models, future efforts should be directed toward examining reproductive experience after age 35 and toward incorporating information on characteristics of the fertile period as it becomes known.

Normative study of age variation in salivary progesterone profiles

Daily luteal progesterone levels were measured for 124 regularly menstruating women, aged 18-44, by radioimmunoassay of steroid levels in saliva. A consistent pattern of age variation in luteal function was found, with the lowest levels of progesterone in the 18-19-year-old and 40-44-year-old groups, the highest values in the 25-34 year olds, and intermediate values in the 20-24 and 35-39-year-old groups. The striking similarity of this pattern of age variation with empirically and theoretically generated curves of apparent fecundability, suggests that age-related changes in ovarian function may underlie observed age variation in fecundability.

Impact of tubal sterilization and vasectomy on female marital sexuality: results of a controlled longitudinal study

To determine if female or male sterilization affects long-term female marital sexuality, we prospectively compared baseline data and 5 consecutive years of follow-up data from 152 tubal sterilization women, 106 vasectomy wives, and 83 women not planning sterilization. By follow-up year 5, no group of women expressed any change in satisfaction with their own sexual response; however, all groups showed a significant decrease across time in satisfaction with their sexual relationship, in coital desire, and in coital frequency. There were no group differences in overall net changes or in rates of change over the 5-year period. However, two short-term group differences were noted: a decrease in coital desire among women not planning sterilization between baseline and follow-up year 4, compared with increases for both sterilization groups, and an increase in coital frequency at the first follow-up year only in the tubal sterilization group. These data indicate that there are no detrimental effects and some short-term benefits of both sterilization procedures on female marital sexuality.

Coitus-induced ovulation and its implications for estimates of some reproductive parameters

It is estimated here that, at the time of conception, the mean coital rate of young women who subsequently bear DZ twins is about 6% higher than that of the general population of young married women. This differential seems too small to account wholly for the greater promptness in conceiving DZ twins. Accordingly, it is suggested that coital rate is associated with DZ twinning in two ways. The major link seems to be indirect and mediated by erotic response which causes an increase in gonadotrophin levels and thus in double ovulation. In a minority of cases, frequent coitus may give rise to DZ twins via superfecundation, but it seems that, in human beings, the corpus luteum of one ovum will usually inhibit fertilization of any further ova after a short interval. The evidence presented here, though indirect, seems to suggest that, under particularly erotic conditions, double ovulation is sometimes induced. It seems reasonable to infer that coitus also occasionally provokes or accelerates single ovulations, thus impugning some rhythm methods of contraception. But there seems no very good evidence that rape induces ovulation. Some notes are added on the implications of induced ovulation for estimates of fecundability and of the length of the fertile period.

Coital frequency and twinning

In a comparative study to enquire whether parents of twins, especially of dizygotic twins, have a higher frequency of sexual intercourse than parents of singleton infants, data on sociodemographic status, coital frequency and other variables were collected using a postal questionnaire. Parents of all twins born alive in Denmark in 1984 or 1985 were included as cases and a random sample of parents of singleton infants born in the same period were controls. No evidence of any difference in coital frequency was found between parents of twins (neither dizygotic twins nor monozygotic) and parents of singleton infants.

Hormonal control of sex ratio

It has been hypothesized that the maternal gonadotrophin level at the time of conception is causally related to the sex of the resultant human zygote, high levels of hormone being associated with the production of female offspring. In this note, evidence for and against this hypothesis is reviewed. There seems so much evidence in its favour that one might conclude that it contains at least a kernel of truth. It is suggested here that of the components of gonadotrophin, the active one in this hypothesized sex-selective process is luteinizing hormone, rather than follicle-stimulating hormone. The hypothesis is nevertheless unable to accommodate several well-established sets of data. Accordingly it is suggested that other hormones, oestrogen and testosterone, have sex-selective properties too, high levels being associated with male offspring. This elaboration of the hypothesis, if it were true, would explain most, if not all, of the epidemiological data on the human secondary sex ratio. In particular it would explain Guerrero's data which have hitherto resisted explanation of any kind. No suggestion is made about possible mechanisms underlying these hypothesized sex-selective properties. But it seems that sex-selective abortion is not the only one. It is hoped that other workers may be stimulated not only to test the hypothesis as outlined here, but--if it survives this testing--to suggest such mechanisms.