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'.

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.

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)