Students of the problem of sex differences in intelligence will be indebted to Mackintosh and Flynn for providing useful further clarification of the issue of sex differences in intelligence. I have little to say about Flynn's paper except to note that it confirms my thesis that among adults males have a small advantage over females in abstract reasoning ability. Mackintosh and I have some common ground but there remain substantial disagreements.

Murphy et al. (1997) showed age-standardised twinning rates for Scotland and England & Wales 1952–94. The rates declined to a value of about 9·2 per 1000 around 1978–81 and subsequently increased to about 11·5 per 1000 in 1992–94. The authors conclude their paper with the words: 'perhaps 15% of twins nationally now follow treatment and the natural twinning rate might still be in decline'.

Dr A. Bener and colleagues have reported socio-demographic risk factors associated with low birthweight in United Arab Emirates by conducting a case-control study.

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'.

We would like to thank Dr K. Nagaraj for his comments on our article and would like to respond and clarify a few points.