O brother, where art thou? The fraternal birth-order effect on male sexual orientation

Sexual Orientation in Twins: Evidence That Human Sexual Identity May Be Determined Five Days Following Fertilization

Human same-sex sexual attraction has been recorded from the beginning of written history. It remains a controversial topic, but recent theories favor prenatal influences. A paradox is the occurrence of same-sex orientation in twins in that there is a higher level of concordance in monozygous twins compared to that in dizygous twins or non-twin siblings. If sexual orientation was entirely genetically determined monozygous twins would be expected to have identical sexual inclinations. Monozygous twins have twice the incidence of sexual concordance in comparison to dizygous twins but a third of these pairs have different sexual identities. An explanation for this disparity may lie in the time an embryo splits to form two separate fetuses. If splitting occurs early in twin development each twin may develop his or her own sexual identity; splitting occurring later results in twins that have the same sexual dispositions. A possible process for such determination may be in the mitochondria, with universal maternal inheritance of a proportion of normal functioning but alternate mitochondria. Variation in the distribution of these mitochondria in neural precursor cells becomes a mechanism for the development of intrinsic sexual orientation and for the spectrum of human sexual inclinations. The timing of embryonic splitting may be determined from the examination of fetal membranes, and the concept of early fetal sexual orientation is open to support or disproval.

Social determinants of syphilis in South China: the effect of sibling position on syphilis and sexual risk behaviours

OBJECTIVE

This study evaluated the relationship between sibling position and sexual risk based on behavioural and syphilis infection data from sexually transmitted infection (STI) patients in South China.

DESIGN

A cross-sectional study examining sexual behaviours and syphilis infection.

SETTING

4 STI clinics in the Pearl River Delta of South China.

PARTICIPANTS

1792 Chinese men and women attending STI clinics.

PRIMARY OUTCOME MEASURES

STI history, syphilis infection defined as positive non-treponemal and treponemal tests.

RESULTS

Among all clinic patients, 824 (46.3%) were first-born, 354 (19.9%) were middle-born and 602 (33.8%) were final-born. Middle-born individuals had a higher percentage of reported STI history (44.7% compared to 34.7%, p<0.001) and syphilis infection (9.7% compared to 4.9%, p=0.01) among men (n=1163) compared to other sibling positions in bivariate analyses, but not in the final multivariate model. The relationship between sibling position and syphilis was independent of income and education level. There was no trend observed between middle-born position and female sexual risk behaviours (n=626). Higher education was significantly associated with syphilis among women and men in respective multivariate models.

CONCLUSIONS

This study suggests that middle-born men in China may have an increased sexual risk compared to other sibling positions. As Chinese family and social structures change, a more thorough understanding of how demographic factors influence sexual risk behaviours is needed.

Influence of the mother's preceding pregnancies on fetal development and postnatal survival of the neonate, in normal pregnancy. An immunological phenomenon?

OBJECTIVES

The objective of this study is to test for an association between the sex of conceptuses of the mother's preceding pregnancies and fetal development and early neonatal survival in normal pregnancy.

METHODS

A population of 27,243 neonates, including a subsample of 7,773 "newborn/mother/placenta units" were divided into cohorts according to the sex of the neonate and the sex and number of conceptuses of the mother's preceding pregnancies. The average birth weight, placenta weight and early neonatal mortality rate were measured for each cohort and compared. The "dose effect" of preceding pregnancy was tested by linear and quadratic regression analysis, and by chi-square trend test for linearity of proportions.

RESULTS

The results have shown an association between these three variables and the preceding pregnancies of the mother. Fetal development and early survival of the neonate are positively associated with the mother's preceding pregnancies of same sex as the neonate, and negatively associated with the preceding pregnancies of opposite sex to the neonate. The strength of the phenomenon increases with parity, at least for the first three parities. The association is statistically significant.

CONCLUSIONS

The association between fetal development and neonatal survival and preceding pregnancies of the mother would be compatible with the action of male and female specific antigens capable of affecting selective implantation of blastocysts, which commands subsequent fetal development as well as early neonatal survival.