PubMed · 6030204
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H H Perlman. 1967. A note on sibling.. https://doi.org/10.1111/j.1939-0025.1967.tb01080.x
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OBJECTIVE: To investigate the incidence of twin and twin with birth defect (BD) and with neural tube defect (NTD) in China. METHODS: A prospective monitoring was conducted among the perinatal infants born in 945 hospitals in 20 provinces/municipalities/autonomous regions in China during the period from 1 October 1986 to 30 September 1987 to count up the numbers of twins and of twins with BD or NTD. RESULTS: 1 243 284 infants were born during this period. Among these perinatal infants there were 12 715 pairs of twins with an incidence of 10.23%. The incidence rates of twin with BD and of twin with NTD were 36.81% and 5.27% respectively. There was no difference in incidence rate of twin among different parts of the country and between urban area and rural area. The incidence rate of twin was the highest among the women aged 20 approximately 39, among primipara, and in October and November. The incidence rate of twin with BD showed no difference among different seasons, parities, and parts of the country and between urban area and rural area. Most of twin with BD occurred at the age 20 approximately 39. Most twins with NTD were born in the northern provinces and rural area. The incidence rate of twin with NTD was correlated with season, most twins with NTD being born in November, and was no correlated with age and parity of the mother. CONCLUSION: The incidence of BD, especially the incidence of NTD, among twins is significantly higher than that among total perinatal infants in China. Prenatal monitoring is important for twin pregnancy.
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In men, sexual orientation correlates with the number of older brothers, each additional older brother increasing the odds of homosexuality by approximately 33%. However, this phenomenon, the fraternal birth order effect, accounts for the sexual orientation of only a proportion of gay men. To estimate the size of this proportion, we derived generalized forms of two epidemiological statistics, the attributable fraction and the population attributable fraction, which quantify the relationship between a condition and prior exposure to an agent that can cause it. In their common forms, these statistics are calculable only for 2 levels of exposure: exposed versus not-exposed. We developed a method applicable to agents with multiple levels of exposure--in this case, number of older brothers. This noniterative method, which requires the odds ratio from a prior logistic regression analysis, was then applied to a large contemporary sample of gay men. The results showed that roughly 1 gay man in 7 owes his sexual orientation to the fraternal birth order effect. They also showed that the effect of fraternal birth order would exceed all other causes of homosexuality in groups of gay men with 3 or more older brothers and would precisely equal all other causes in a theoretical group with 2.5 older brothers. Implications are suggested for the gay sib-pair linkage method of identifying genetic loci for homosexuality.