Birth order and sexual orientation in men: evidence for two independent interactions

Birth order is correlated with male sexual orientation, but the reason(s) for this relationship is unclear. In the present study, data from a Canadian sample of homosexual and heterosexual men (N=604) were used to present evidence of two independent birth order interactions--one with height and the other with parental age--predicting sexual orientation in men. If these findings prove reliable, it raises the possibility that different aetiological factors underlie the birth order/sexual orientation relationship in men.     

The relationship between parental age and birth order with the percentage of low birth-weight infants

Birthweight, birth order, and parental age were abstracted from 1,515,443 New York State birth certificates to study the association between the birth of an infant weighing less than 2501 g and parental age. The percentage of premature infants was greatest for birth order 1 and 6+ and showed a minimum at birth order 3. When maternal age and birth order were analyzed jointly, a strong interaction was found. Young mothers showed a tendency to have an increasing proportion of low birthweight infants with increasing birth order, whereas, the exact opposite was true for mothers older than 45. The intermediate maternal age categories reflected this change from an association of increasing proportion of low birthweight infants with increasing birth order to a pattern of decreasing proportion of premature imfants with increasing birth order. In data stratified to eliminate the influence of maternal age and to some extent birth order, paternal age was shown to affect the percentage of infants weighing less than 2501 g. This association was described by a flat n-shaped curve that was significantly different from a horizontal line (P.01) in 6 of 7 maternal age categories.     

The association between the fraternal birth order effect in male homosexuality and other markers of human sexual orientation

Later fraternal birth order (FBO) is a well-established correlate of homosexuality in human males and may implicate a maternal immunization response in the feminization of male sexuality. This has led to the suggestion that FBO may relate to other markers of male sexual orientation which are robustly sexually dimorphic. If so, among homosexual males the number of older brothers should strongly correlate with traits such as spatial ability and psychological gender, indicative of greater behavioural feminization, compared to heterosexual males. The present study failed to find significant associations between number of older brothers and these traits.     

Correlation of age and birth order of parents with chromosomal anomalies in children

One hundred children with suspected congenital and/or malformation and their parents who reported to SAT hospital, Medical College, Trivandrum, India formed the test group. Fifty children with no obvious anomalies or abnormalities and their parents formed the control group. The criteria for selection of the control was 1) the maternal age at delivery was below 30 years and 2) the parents belong to 1st or 2nd birth order. The chromosomal analysis was carried out in all the subjects using peripheral blood lymphocyte microculture to investigate for any constitutional chromosomal markers and quantitate the mutagen (bleomycin) sensitivity of the chromosomes. All the subjects were evaluated clinically and a complete family history was recorded. Chromosome anomalies were noted in 41 out of the one hundred children and in 4 out of the 200 parents of the test group. No constitutional aberrations were seen either in the parents or in the children of the control group. Bleomycin sensitivity study revealed a high b/c value in 35 children (24 hypersensitive and 11 sensitive) of the test group whereas in the control group the b/c values were low denoting hyposensitivity and very good DNA repair mechanism. This study reveals that the incidence of chromosome aberrations is higher when the age and birth order of parents are higher. A direct correlation was noted with parental order and b/c value. This was also true with the parental age and birth defects.     

Distribution of age at menopause in two Danish samples

We analyzed the distribution of reported age at natural menopause in two random samples of Danish women (n = 176 and n = 150) to determine the shape of the distribution and to disclose any possible trends in the distribution parameters. It was necessary to correct the frequencies of the reported ages for the effect of differing ages at reporting. The corrected distribution of age at menopause differs from the normal distribution in the same way in both samples. Both distributions could be described by a mixture of two normal distributions. It appears that most of the parameters of the normal distribution mixtures remain unchanged over a 50-year time lag. The position of the distribution, that is, the mean age at menopause, however, increases slightly but significantly.     

Independent effects of maternal age and birth order on the incidence of selected congenital malformations

Incidence rates, specific for maternal age and birth order, were calculated for 16 categories of congenital malformations reported on birth certificates from a population of more than 8 million registered, white, single livebirths. With maternal age held constant, none of the malformations showed increasing incidence as birth order increased. Hypospadias, esophageal defects, omphalocele, and Down syndrome showed evidence of decreasing incidence as birth order increased. Some of the other malformation categories showed an excess of 1st births in most age groups, while no relation to birth order was observed in the incidence of other malformations. By contrast, most of the malformations analyzed exhibited increasing incidence as maternal age increased. Especially high rates of several malformations were observed among 1st births to women over age 40.     

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.     

The influence of the parental environment on sexual maturation in male mice

Experiments were designed to examine the influence of either one or both parents on the sexual maturation of male mice. A pregnant female and an adult vasectomized male were housed together and, at delivery, the litter was reduced to 6 pups, 3 of each sex. After weaning, mother, foster-father or both, stayed in cohabitation with their pups until sacrifice. The results showed that: (1) the permanent presence of one or both parents did not affect either testicular growth or the age at first fertile matings. (2) in maturing males (30-40 days), seminal vesicle growth was accelerated by the presence of the mother, inhibited by the presence of the foster-father. (3) in maturing males, the presence of one or both parents lowered plasma testosterone levels. On the opposite, in 90-day-old males, the same conditions increased testosterone levels. These results indicate that only the endocrine function of the testis is affected by the parental environment. The effects are age-dependent.     

The evolution of parental care in the context of sexual selection: a critical reassessment of parental investment theory

Males and females are often defined by differences in their energetic investment in gametes. In most sexual species, females produce few large ova, whereas males produce many tiny sperm. This difference in initial parental investment is presumed to exert a fundamental influence on sex differences in mating and parental behavior, resulting in a taxonomic bias toward parental care in females and away from parental care in males. In this article, we reexamine the logic of this argument as well as the evolutionarily stable strategy (ESS) theory often used to substantiate it. We show that the classic ESS model, which contrasts parental care with offspring desertion, violates the necessary relationship between mean male and female fitness. When the constraint of equal male and female mean fitness is correctly incorporated into the ESS model, its results are congruent with those of evolutionary genetic theory for the evolution of genes with direct and indirect effects. Male parental care evolves whenever half the magnitude of the indirect effect of paternal care on offspring viability exceeds the direct effect of additional mating success gained by desertion. When the converse is true, desertion invades and spreads. In the absence of a genetic correlation between the sexes, the evolution of paternal care is independent of maternal care. Theories based on sex differences in gametic investment make no such specific predictions. We discuss whether inferences about the evolution of sex differences in parental care can hold if the ESS theory on which they are based contains internal contradictions.     

The stress effect in the prenatal period on sexual excitation and sexual orientation of the mice males

Estrus female behind holed transparent partition produced sexual motivation and sexual arousal in males. It was manifested in behavioral changes (an increase in time spent near the partition) and the testosterone level augmentation in blood. Female mice were exposed to stress (1 h/day restraint) in the last week of gestation. Prenatal stress was shown to decrease the blood corticosterone level as well as to diminish sexual motivation and sexual arousal in adult male mice. Estrus female exposure produced a lesser behavioral response and a lesser testosterone level augmentation. No changes in weight of testicles, seminal vesicles or adrenal glands were found, but preputial gland weight increased. In prenatally stressed males, a female preference decrease and a male preference increase were revealed in the partner preference test. These data suggest that prenatal stress decreases sexual motivation in males and leads to clear predisposition to homosexuality, although it does not produce complete inversion of sexual orientation.     

First sexual intercourse: age,coercion, and later regrets reported by a birth cohort.

OBJECTIVES: To investigate how age at first sexual intercourse is related to the reported circumstances and to determine how these corresponded to views in early adulthood about its timing. DESIGN: Cross sectional study within a birth cohort using a questionnaire presented by computer. SETTING: Dunedin, New Zealand in 1993-4. SUBJECTS: 477 men and 458 women enrolled in the Dunedin Multidisciplinary Health and Development Study, comprising 92% of survivors of the cohort. RESULTS: The median age at first intercourse was 17 years for men and 16 years for women. Only one man (0.2%) but 30 (7%) women reported being forced to have intercourse on the first occasion. For women, there were increasing rates of coercion with younger age at first intercourse. More men than women reported that they and their partner were equally willing (77% (316/413) v 53% (222/419)). Mutual willingness of both partners was greater for those who reported that it was also the first time for their partner. Timing of first intercourse was considered about right by 49% (200/411) of men and 38% (148/388) of women. Many women (54% (211/388) reported that they should have waited longer, and this rose to 70% (90/129) for women reporting intercourse before age 16. CONCLUSIONS: Most women regretted having sexual intercourse before age 16. First intercourse at younger ages is associated with risks that are shared unequally between men and women. This information is important to young people themselves.