Effect of producing sons on maternal longevity in premodern populations

Infant sex at birth and long-term maternal mortality

BACKGROUND

Maternal adaptations may vary by foetal sex. Whether male infants influence long-term mortality in mothers remains uncertain.

OBJECTIVE

The objective of the study was to examine whether male infants increase the risk of maternal mortality.

METHODS

This study included pregnant women enrolled at 12 US sites from 1959 to 1966 in the Collaborative Perinatal Project (CPP). Collaborative Perinatal Project records were linked to the National Death Index and the Social Security Master Death File to ascertain deaths until 2016. Foetal sex was determined by infant sex at birth, defined as the total number of male or female infants in pregnancies prior to or during enrolment in the CPP. In secondary analyses, exposure was defined as infant sex at the last CPP delivery. Outcomes included all-cause and underlying causes of mortality. We used Cox proportional hazards models weighted by the number of prior live births and stratified our models by parity and race/ethnicity.

RESULTS

Among 48,188 women, 50.8% had a male infant at their last registered CPP pregnancy and 39.0% had a recorded death after a mean follow-up of 47.8 years (SD 10.5 years). No linear association was found between the number of liveborn males and all-cause mortality (primipara women: HR 1.02, 95% CI 0.95, 1.09, multipara women, 1 prior live birth: HR 0.96, 95% CI 0.89, 1.03, multipara women, ≥2 prior live births: HR 0.97, 95% CI 0.85, 1.11). A similar trend was noted for cardiovascular- and cancer-related mortality. At the last delivery, women with a male infant did not have an increased risk of all-cause or cause-specific mortality compared to women with a female infant. These findings were consistent across racial/ethnic groups.

CONCLUSIONS

Women who give birth to male infants, regardless of number, are not at increased risk of all-cause and cause-specific mortality. These findings suggest that giving birth to male infants may not independently influence the long-term health of women.

Impact of fertility on the longevity of older rural Chinese women: an analysis of a longitudinal survey

Background

This study evaluates the impact of fertility during the childbearing period on the longevity of older rural Chinese women and verifies whether any trade-off exists between women’s longevity and their number of children to provide empirical evidence for improving health intervention policies and formulating active fertility policies in low-fertility countries.

Methods

Based on the data of the deaths of 1623 older adults aged 65 and above during 2014–2018 in the Chinese Longitudinal Healthy Longevity Survey, this study explores the relationship between the number of children born and older rural women’s longevity using the ordinary least squares method. Furthermore, the impact of fertility on the longevity of men and women in rural and urban areas, along with other reproductive behaviours on older rural women’s longevity, were analysed.

Results

There was a significant negative correlation between the number of children born and women’s longevity (β = − 0.555, p < 0.05). Additionally, their longevity exhibited a decreasing trend with having birthed more sons and an increasing trend with more daughters. Age at first and last births had a significant positive relationship with rural women’s longevity; however, the effect of fertility on the longevity of older rural and urban men and older urban women was not significant.

Conclusions

It is confirmed that there is a trade-off between fertility and longevity for rural women in China. Future research should focus on compensating for the decline in female longevity caused by the number of children born and promote the concept of a healthy pregnancy, scientific nurture, and gender equality in fertility.

The Influence of Female Reproductive Factors on Longevity: A Systematized Narrative Review of Epidemiological Studies

Purpose: This systematized review presents a synthesis of epidemiological studies that examine the association between female reproductive factors and longevity indicators. Methods: A comprehensive literature search was conducted using four bibliographic databases: OVID Medline, Web of Science, PubMed, and Google Scholar, including English language articles published until March 2022. Results from the search strategy yielded 306 articles, 37 of which were included for review based on eligibility criteria. Results were identified within the following nine themes: endogenous androgens and estrogens, age at first childbirth, age at last childbirth, parity, reproductive lifespan, menopause-related factors, hormone therapy use, age at menarche, and offspring gender. Results: Evidence that links reproductive factors and long lifespan is limited. Several female reproductive factors are shown to be significantly associated with longevity, yet findings remain inconclusive. The most consistent association was between parity (fertility and fecundity) and increased female lifespan. Age at first birth and parity were consistently associated with increased longevity. Associations between age at menarche and menopause, premature menopause, reproductive lifespan, offspring gender and longevity are inconclusive. Conclusion: There is not enough evidence to consider sex a longevity predictor. To understand the mechanisms that predict longevity outcomes, it is imperative to consider sex-specific within-population differences.

The Relationship Between Children's Gender Composition and Parents' All-Cause Mortality Among Older Adults in Korea

Using data from the Korean Longitudinal Study of Aging (N = 3,356 participants aged 65 or older) and estimating Cox proportional hazard regression models, this study investigated the association between gender composition of children and parents' mortality risk. This study showed that compared with having both daughters and sons, having exclusively daughters (but not exclusively sons) was associated with higher mortality risk. This association, however, was significant for mothers, but not for fathers. To shed light on source of this association, this study distinguished between mothers with only daughters who receive and do not receive regular financial support from children. Results showed that mothers had elevated mortality risk only if they do not receive regular financial support from their children. Traditional caregiving roles of sons for older parents in Korea may lead to a mortality disadvantage among mothers of daughters vis-à-vis a reduction in financial assistance from children.

Inflammaging: Blame the sons. Relationships between the number of sons and the level of inflammatory mediators among post-reproductive women

OBJECTIVES

Reproduction is costly, but sons and daughters differently influence maternal physiology, also in older age. In particular, having sons may negatively influence maternal health and may be associated with a shorter life span of mothers. Sons may also contribute to increased inflammaging, a chronic sub-clinical systemic inflammatory state characterized by elevated levels of serum inflammatory mediators. The aim of this study was to examine the impact of the total number of children, and the number of daughters and sons separately on concentrations of C-reactive protein (CRP), and proinflammatory cytokines such as interleukin 6 (IL-6) and tumor necrosis factor-α (TNF-α).

MATERIALS AND METHODS

The participants were 378 women aged 45-92 who had 3.9 (SD 2.12, median = 4, range = 0-13) children, including 2.1 (SD 1.46, median = 2, range = 0-8) sons and 1.8 (SD 1.44, median = 2, range = 0-7) daughters on average.

RESULTS

We found a positive relationship between the overall number of children and IL-6 levels. CRP and IL-6 concentrations were positively associated with the number of sons but not with the number of daughters. Each son increased maternal CRP level by 11%, and IL-6 level by 6%. Neither the total number of children nor the number of daughters or sons were related to the TNF-α concentration.

DISCUSSION

Aging-associated inflammation in post-reproductive mothers with a higher number of sons supports the hypothesis of trade-offs between reproduction and health. Furthermore, these results provide new evidence contributing to the idea that having sons may have more detrimental effects on the maternal organism than having daughters.

Accelerated senescence as a cost of reproduction: Testing associations between oxidative stress and reproductive effort in rural and urban women

OBJECTIVES

Oxidative stress is hypothesized to contribute to age-related somatic deterioration. Both reproductive and ecological context may necessitate tradeoffs that influence this outcome. We examined whether measures of lifetime reproductive effort were related to levels of oxidative stress biomarkers in peri- and post-menopausal women and whether associations were moderated by rural or urban residence.

METHODS

We surveyed 263 healthy women (age 62.1 ± 10.0 SD) from rural (N = 161) and urban Poland (N = 102), collecting sociodemographic data and urine samples to analyze biomarkers of oxidative stress (8-oxo-2'-deoxyguanosine, 8-OHdG) and antioxidative defense (copper-zinc superoxide dismutase, Cu-Zn SOD). Linear regression models, adjusted for residence, were used to test for associations between reproductive effort and 8-OHdG and Cu-Zn SOD.

RESULTS

Univariate models demonstrated significant associations between gravidity and the biomarkers of oxidative stress (8-OHdG: R² = 0.042, P ≤ .001; Cu-Zn SOD: R² = 0.123, P ≤ .001). Multivariate models incorporating potential confounding variables, as well as cross-product interaction terms, indicated that gravidity was associated with 8-OHdG (P < .01, R² _adj = 0.067) and Cu-Zn SOD (P = .01, R² _adj = 0.159). Residence (ie, urban vs rural) did not significantly moderate the associations between the biomarkers and reproductive effort.

CONCLUSIONS

Higher lifetime reproductive effort contributes to increases in oxidative stress and antioxidative defenses. Our results provide evidence of potential mechanisms underlying the physiological tradeoffs influencing senescence for women with high reproductive effort. We illustrate the value of applying an evolutionary perspective to elucidate variation in human health and senescence.

Sons accelerate maternal aging in a wild mammal

Aging, or senescence, is a progressive deterioration of physiological function with age. It leads to age-related declines in reproduction (reproductive senescence) and survival (actuarial senescence) in most organisms. However, senescence patterns can be highly variable across species, populations, and individuals, and the reasons for such variations remain poorly understood. Evolutionary theories predict that increases in reproductive effort in early life should be associated with accelerated senescence, but empirical tests have yielded mixed results. Although in sexually size-dimorphic species offspring of the larger sex (typically males) commonly require more parental resources, these sex differences are not currently incorporated into evolutionary theories of aging. Here, we show that female reproductive senescence varies with both the number and sex ratio of offspring weaned during early life, using data from a long-term study of bighorn sheep. For a given number of offspring, females that weaned more sons than daughters when aged between 2 and 7 y experienced faster senescence in offspring survival in old age. By contrast, analyses of actuarial senescence showed no cost of early-life reproduction. Our results unite two important topics in evolutionary biology: life history and sex allocation. Offspring sex ratio may help explain among-individual variation in senescence rates in other species, including humans.

Maternal longevity and offspring sex in wild ungulates

In species with sexual size dimorphism, the offspring of the larger sex usually have greater energy requirements and may lead to greater fitness costs for parents. The effects of offspring sex on maternal longevity, however, have only been tested in humans. Human studies produced mixed results and considerable debate mainly owing to the difficulty of distinguishing the effects of sexual dimorphism from sociocultural factors. To advance this debate, we examined how the relative number of sons influenced maternal longevity in four species of free-living ungulates (Soay sheep Ovis aries; bighorn sheep, Ovis canadensis; red deer, Cervus elaphus; mountain goat, Oreamnos americanus), with high male-biased sexual size dimorphism but without complicating sociocultural variables. We found no evidence for a higher cumulative cost of sons than of daughters on maternal longevity. For a given number of offspring, most females with many sons in all four populations lived longer than females with few sons. The higher cost of sons over daughters on maternal lifespan reported by some human studies may be the exception rather than the rule in long-lived iteroparous species.

Offspring sex and parental health and mortality

Increased mortality has been observed in mothers and fathers with male offspring but little is known regarding specific diseases. In a register linkage we linked women born 1925–1954 having survived to age 50 (n = 661,031) to offspring and fathers (n = 691,124). Three approaches were used: 1) number of total boy and girl offspring, 2) sex of the first and second offspring and 3) proportion of boys to total number of offspring. A sub-cohort (n = 50,736 mothers, n = 44,794 fathers) from survey data was analysed for risk factors. Mothers had increased risk of total and cardiovascular mortality that was consistent across approaches: cardiovascular mortality of 1.07 (95% CI: 1.03–1.11) per boy (approach 2), 1.04 (1.01–1.07) if the first offspring was a boy, and 1.06 (1.01–1.10) if the first two offspring were boys (approach 3). We found that sex of offspring was not associated with total or cardiovascular mortality in fathers. For other diseases or risk factors no robust associations were seen in mothers or fathers. Increased cardiovascular risk in mothers having male offspring suggests a maternal disease specific mechanism. The lack of consistent associations on measured risk factors could suggest other biological pathways than those studied play a role in generating this additional cardiovascular risk.

Fertility History, Children's Gender, and Post-Reproductive Survival in a Longevous Population

The question of whether mothers' fertility history influences their post-reproductive survival has been addressed frequently in the scientific literature. Using data from Villagrande Strisaili, Sardinia, where longevity is higher than anywhere else in Europe, we analyzed the relationship between the fertility pattern of mothers who survived past age 50 (n = 539) and their post-reproductive lifespan. We find that, after adjustment for potential confounders (mothers' birth cohort, survival of spouse), the mothers who on average delivered their children later displayed a reduced mortality risk (‒2.9 percent for each additional year), supporting previously reported findings. We also find that a male-skewed offspring ratio was associated with decreased mortality risk of mothers, with longer survival of mothers who delivered their sons above age 35 (p = 0.005), a result not found for daughters. So far, no biological explanation has been suggested for the positive effect of delivering sons later in life. We conjecture that in our dataset stronger nonbiological factors such as gender-specific sociocultural and economic factors may have masked the negative effect reported in other populations, for which a biological explanation was proposed.

The impact of children's sex composition on parents' mortality

BACKGROUND

This study explores the relationship between children's sex composition and parents' mortality in a contemporary western society. It improves on earlier research by using a larger and more representative dataset - constructed from registers and encompassing the entire Norwegian population.

METHODS

The analysis is based on discrete-time hazard models, estimated for the years 1980-2008 for women and men born after 1935.

RESULTS

When operationalising sex composition as the "number of boys", coefficients are insignificant in all specifications. However, when considering the three categories "only boys", "only girls" and "mixed sex", I find a small but significant disadvantage of having only girls, compared to having at least one child of each sex, for mothers of two or more children. Having only daughters is associated with a mortality disadvantage compared to having only sons for mothers of two children, but a mortality advantage among mothers with four children. Among women who gave birth to their first child as teenagers, those who have only sons have relatively high mortality. I also find an excess mortality both for mothers of only girls and mothers of only boys in the period 1980-1989.

CONCLUSION

These results lend some support to the notion that there is a larger benefit of the first son or daughter than the later children of the same sex, and especially in the earliest decade of the study period.

Reexamining the association between fertility and longevity: testing the disposable soma theory in a modern human sample of twins

The disposable soma theory proposes a trade-off between fertility and longevity but existing findings on this association have been mixed. This study used data from 15,622 twins born between 1901 and 1925 ascertained from the population-based Swedish Twin Registry to test the child-longevity association and whether it is accounted for by individual-level factors or by genetic and environmental factors shared by family members. Based on survival analysis, both women and men with children had significantly longer survival relative to the childless, with a slightly higher relative advantage in men. Adjustments for demographic factors and cotwin fertility did not mediate the parenting-survival association, indicating that this association is attributable to individual-level factors associated with fertility rather than family-level environmental or genetic factors shared by cotwins. These results, derived from a large, population-based sample, are inconsistent with the disposable soma theory as applied to modern human populations.

Widowhood, illegitimacy, marital reproduction and female longevity in a rural Spanish population

Using data from parish and civil registers in a rural community in northwest Spain (Los Nogales), family reconstitution provided 1502 complete reproductive histories, of which 584 corresponded to first marriages of women dying after their 50th birthday. A homogeneous sample consisting of women married in the period 1877-1899 (N=311) provided information concerning their reproductive performance, including ages at first and last maternity and number of children born alive and surviving, which was related to the mother's post-menopausal longevity, also considering premarital fertility and her marital status (widow/married). The results obtained indicate that mothers with a lower proportion of children dying before the first birthday and the age of 15 (mainly males) have a greater post-reproductive longevity. Moreover, women with a more protracted end to their reproductive period and greater fertility live for more years beyond their 50th birthday. These results do not prove a causality between maternal longevity and more successful reproduction; instead, they are indicative of a holistic condition of health. A wide spectrum of favorable biological and environmental factors will have positive consequences for a woman's life trajectory, affecting both her reproductive performance and her own likelihood of surviving.

Do sons reduce parental mortality?

BACKGROUND

Although sons are thought to impose greater physiological costs on mothers than daughters, sons may be advantageous for parental survival in some social contexts. The authors examined the relationship between the sex composition of offspring and parental survival in contemporary China and Taiwan. Because of the importance of sons for the provision of support to elderly parents in these populations, the authors hypothesised that sons would have a beneficial effect on parental survival relative to daughters.

METHODS

The authors used data from the Chinese Longitudinal Healthy Longevity Survey (CLHLS) and the Taiwan Longitudinal Study of Aging (TLSA). The CLHLS sample consisted of 4132 individuals aged 65 years and over in 2002. The TLSA sample comprised two cohorts: 3409 persons aged 60 years and over in 1989 and 2193 persons aged 50-66 years in 1996. These cohorts were followed up for 3, 18 and 11 years, respectively. The Cox proportional hazards model was used to estimate the relationship between the sex composition of offspring and parental mortality.

RESULTS

Based on seven measures of sex composition, no protective effect of sons was found in either China or Taiwan. For example, in the 1989 Taiwan sample, the hazard ratio (HR) for maternal mortality associated with having an eldest son was 0.979 (95% CI 0.863 to 1.111). In Taiwan, daughters may have been more beneficial than sons in reducing mortality in recent years.

CONCLUSION

The authors offer several explanations for these findings, including possible benefits associated with emotional and interpersonal forms of support provided by daughters and negative impacts of conflicts arising between parents and resident daughters-in-law.

Parity and parents' health in later life: the gendered case of Ismailia, Egypt

We investigate the relationship between reproduction and functional health in later life among women and men in the resource-poor and gender-stratified setting of Ismailia governorate, Egypt. Analyses of survey data collected in 2003 show a statistically significant positive association between parity and difficulty with activities of daily living (ADLs), controlling for demographic and socio-economic factors and other co-morbid conditions. We also find that the number of daughters (but not sons) is associated with worse physical functioning, and this association is more pronounced for older fathers than for older mothers. Our results indicate that both biological and social pathways link fertility and later-life health in this context, and that prescribed familial roles may underlie the differential impact of sons and daughters on the health of mothers and fathers in later life.

Is there an adverse effect of sons on maternal longevity?

Recent years have witnessed the emergence of a literature examining the effects of giving birth to sons on postmenopausal longevity in pre-industrial mothers. The original paper in this lineage used a sample (n=375) of Sami mothers from northern Finland and found that, relative to daughters, giving birth to sons substantially reduced maternal longevity. We examine this hypothesis using a similar and a much larger sample (n=930) of pre-industrial Sami women from northern Sweden, who in terms of their demographic, sociocultural and biological conditions, closely resemble the original study population. In contrast to the previously reported results for the Sami, we find no evidence of a negative effect of sons on maternal longevity. Thus, we provide the most compelling evidence to date that the leading result in the literature must be approached with scepticism.

Widowhood, family size, and post-reproductive mortality: a comparative analysis of three populations in nineteenth-century Europe

Researchers from a number of disciplines have offered competing theories about the effects oJ childbearing on parents 'postreproductive longevity. The "disposable soma theory" argues that investments in somatic maintenance increase longevity but reduce childbearing. "Maternal depletion" models suggest that the rigors of childrearing increase mortality in later years. Other researchers consider continued childbearing a sign of healthy aging and a predictor of future longevity. Empirical studies have produced inconsistent and contradictory results. Our focus is on the experience of widowhood, which has been ignored in previous studies. We hypothesize that the death of a spouse is a stressful event with long-term consequences for health, especially for women with small children. Data are drawn from historical sources in Sweden, Belgium, and the Netherlands from 1766 to 1980. Postreproductive mortality was highest among young widows with larger families in all three samples. Age at last birth had little or no effect. We conclude that raising children under adverse circumstances can have long-lasting, harmful effects on a mother's health.

Maternal longevity and the sex of offspring in pre-industrial Sweden

BACKGROUND

Helle et al. (2000. Sons reduced maternal longevity in preindustrial humans. Science, 296, 1085) argued that giving birth to sons reduced maternal longevity in pre-industrial societies due to higher physiological costs of bearing sons and the elevated testosterone levels observed in mothers carrying male foetuses.

AIM

The present study examined this hypothesis using a more comprehensive dataset and evaluated the merits of the statistical approach used in previous studies to identify the cost of giving birth to sons in terms of maternal old-age longevity.

SUBJECTS AND METHODS

The analysis in Helle et al. (2002. Sons reduced maternal longevity in preindustrial humans. Science 296, 1085) was extended by using a considerably larger dataset of pre-industrial Swedish women, and with careful consideration paid to methodological problems of sample selection and omitted variable bias. We argue that the previous literature has underestimated the difficulties in quantifying the trade-off between parity and longevity due to unobserved heterogeneity in health. However, under less restrictive assumptions, one can estimate the marginal impact of a son for a fixed family size.

RESULTS

No evidence was found of a negative relative impact of sons. Neither was any evidence found in favour of the male-biased intra-household resource competition hypothesis proposed elsewhere in the literature, despite the poverty of the study population. These results are robust to a wide range of specifications tested.

CONCLUSION

The failure to reproduce earlier findings and the fact that studies in this area of research seem to continue to yield conflicting results warrant much caution in discussing and evaluating results. It is likely that the negative effect of sons, if it existed, only manifested itself under conditions that are not yet fully understood. We also argue that the previous literature on this topic has not fully acknowledged the inference problems associated with omitted variable bias and sample selection.

Does reproduction decrease longevity in human beings?

According to the disposable soma theory, a cost for reproduction could exist in human beings and other species and, thus, longevity could decrease when women have a higher number of children. The purpose of this article is to review the evidence in populations living or not living under natural fertility conditions, i.e. when fertility is near its biological maximum. The results indicate that in natural fertility conditions longevity does not decrease when the number of children increases but, in modern populations, mortality could slightly increase when women have more than ca 5 children. Complete data for these modern cohorts will tell us, one day, whether these results are still observed when the variable of interest is longevity and not only mortality.

The impact of reproduction on gambian women: Does controlling for phenotypic quality reveal costs of reproduction?

Life history theory predicts that where resources are limited, investment in reproduction will cause a decline in body condition and ultimately may lower survival rates. We investigate the relationship between reproduction and mortality in women in rural Gambia. We use a number of different measures of reproductive investment: the timing of reproduction, intensity of reproduction, and cumulative reproductive investment (parity). Though giving birth is clearly a risk factor for increased mortality, we find limited evidence that the timing, intensity, or cumulative effects of reproduction have a survival cost. Instead, there is some evidence that women who have invested heavily in reproduction have higher survival than women with lower reproductive investment: both high parity and late age at last reproduction are associated with high survival. The only evidence for any cost of reproduction is that women who have given birth to twins (considered a marker of heavy investment in reproduction) have higher mortality rates than other women, after the age of 50 years. A potential confounding factor may be differences in health between women: particularly healthy women may be able to invest substantially in both reproduction and their own survival, leading to the positive correlations between survival and both parity and age at last birth we observe. To control for differences in health between women, we reanalyze the relationship between reproduction and mortality but include variables correlating with health in our models (height, BMI, and hemoglobin). Even when controlling for health, the positive correlation between investment in reproduction and survival remains unchanged.

Daughters increase longevity of fathers, but daughters and sons equally reduce longevity of mothers

Reproduction is energetically and physiologically expensive, and an individual investing resources into producing offspring should suffer costs such as deterioration in health condition and possibly shorter life span. Since the energetic and nutritional demands of pregnancy and breastfeeding render reproductive costs much higher in women than in men, women with a large number of children should show signs of deterioration in condition, while men with large families should not. However, whether reproductive costs reduce longevity in women is still questionable, and in men this issue has not been adequately addressed. In addition, since sons are energetically more expensive to produce than daughters, having sons should have a more pronounced negative impact on maternal longevity than having daughters. Here we document a striking disparity in the impact of children on the life span of mothers and fathers in a Polish rural population. We show for the first time that number of daughters was positively related to a longer life span of their fathers, increasing their longevity on average by 74 weeks per daughter born, while number of sons did not have a significant effect on paternal longevity. In contrast, in women, the number of daughters and number of sons reduced maternal longevity and did so to the same extent, on average by 95 weeks per son or daughter, indicating that for women, the costs of having sons and daughters are similar.

Does the number of sons born affect long-term mortality of parents? A cohort study in rural Bangladesh

It has been suggested that bearing sons increases long-term mortality in women, because sons may be more physiologically demanding to produce than daughters. In this historical cohort study in rural Bangladesh, no association between the number of sons born and mortality was seen in women in the unadjusted analyses. However, a significant reduction in mortality with the number of surviving sons was seen. In addition, after adjusting for the number of surviving sons, there was evidence of increasing mortality with the number of sons born, in women. In men, mortality also depended strongly on the number of surviving sons, but not on the number born. These data provide support for negative long-term costs of bearing sons in mothers in rural Bangladesh, and suggest that there are context-specific factors that mask the true effects of sons in some populations.

Long-term effects of childbearing on mortality: evidence from pre-industrial Sweden

This paper presents an analysis of the impact of childbearing history on later-life mortality for ever-married men and women using historical micro-level data of high quality for southern Sweden. The analysis uses a Cox proportional hazards model, estimating the effects on old-age mortality of number of births and timing of first and last births. By studying the effects of previous childbearing on mortality by sex and social status, we also gain important insights into the mechanisms relating childbearing to mortality in old age. The results show that number of children ever born had a statistically significant negative impact on longevity after age 50 for females but not for males. Analysis by social group shows that only landless women experienced higher mortality from having more children, which seems to indicate that the main explanations are to be found in social or economic conditions specific to females, rather than in the strictly biological or physiological effects of childbearing.

Reproduction and longevity among the British peerage: the effect of frailty and health selection

Whether a cost of reproduction exists among humans is still questionable. A major study of aristocratic British families finds a significant positive correlation between parity and late-life mortality, which indicates a trade-off between reproduction and longevity. This result is supported by four other studies, while earlier studies have not found a relationship or came to the opposite conclusion. We show that in natural fertility populations the relationship between fertility and late-life mortality cannot be studied correctly without considering the effects of differences in health and of mortality selection during childbearing ages because these two effects lead to a dampening of the true relationship. If these effects are controlled in Hollingsworth's genealogy of the British peerage a significant trade-off between reproduction and longevity exists for females but not for males.