SummaryIn all European countries for which data exist, there was a maternal-age-specific decline in dizygotic twinning rates during the 1960s. For most of these countries, this decline continued through the 1970s, but in a few it apparently ceased. The causes of the declines and of their abatement are unknown. However, there were declines in sperm quality during the 1960s and 1970s in some parts of the world, including parts of Europe, and it is speculated here that this decline in sperm (...) quality may be related to the decline in dizygotic twinning. (shrink)
Amankwaa (1996) gives data on infant mortality in monogamous and polygynous families. However he does not categorise his data by sex. This is important because one might expect infant mortality to vary by sex; moreover the offspring sex ratios of polygynous and monogamous women reportedly differ.
SummaryIt is usually supposed that androgens play a major role in the aetiology of prostatic cancer. This note suggests that the association of the disease with low coital rates and its apparent association with vasectomy can both be reconciled with the hypothesised androgenic involvement.
It seems that women whose first-born children are very heavy or very light have more subsequent children than other women. This finding does not appear to arise from any of the possible artifacts discussed, and it could have some bearing on the present secular increase in the height and weight of the population.
SummaryA study has been made of twinning rates throughout the world over the last two decades. There has been a decline in age-specific dizygotic twinning rates in almost all of the developed countries during this period. Exceptions to this trend are provided by the United States, where the only decline in twinning rates in the last two decades was in births to older women in the 1960s, and by Czechoslovakia, Hungary and Bulgaria, where an initial decline in twinning during the (...) 1960s was followed by stable twinning rates.It is suggested that the cause may be some form of environmental pollutant, perhaps a pesticide, which has been the subject of restrictive legislation first in the United States and later in the three Communist countries. Possibly the cause of this decline may be identified by a study of such legislation.This seems to be the first study of twinning rates in Central and South America. The low twinning rates in some of the countries there may indicate the genetic affiliations of their inhabitants with those of countries in the Far East. (shrink)
Rao & DeMaris (1995) suggest that when age is controlled, cohabiting partners have higher coital rates than married partners. Their sample was restricted to couples whose relationships were of 5 years' duration or less. This restriction, according to the authors 'circumvents the confounding problem of age with duration of relationship'.
Murphy etal. (1997) showed age-standardised twinning rates for Scotland and England & Wales 195281 and subsequently increased to about 11·5 per 1000 in 1992–94. The authors conclude their paper with the words: 'perhaps 15% oftwins nationally now follow treatment and the natural twinning rate mightstill be in decline'.
SummaryIt is argued that the author's hypothesis relating timing of insemination to the sex ratio of the resulting children is supported by the available data on the variation of sex ratio with sibship size, the presence of twin sibs, and parental adherence to the Jewish faith.