Response to commentaries

I am flattered by the thoughtful commentaries on my paper. ‘Troubled sleep’ had two major purposes. The first was to draw attention to the oppositely perturbed sleep of infants with Prader–Willi syndrome (PWS) and Angelman syndrome (AS) and explore its evolutionary implications. The involvement of imprinted genes suggests that infant sleep has been subject to antagonistic selection on genes of maternal and paternal origin with genes of maternal origin favoring less disrupted sleep. McKenna [1] is uncomfortable with the notion that night waking depends on a single gene and believes that I discount a significant role for culture. I did not intend to imply that the control of infant sleep was simple. Its pattern will be determined by complex interactions among genes, by cultural practices and by negotiations between individual caregivers and infants. My second major purpose was a critique of the idea that children would be happier, healthier and better-adjusted if we could only return to natural methods of child care. This way of thinking is often accompanied by a belief that modern practices put children at risk of irrevocable harm. The truth of such claims is ultimately an empirical question, but the claims are sometimes presented as if they had the imprimatur of evolutionary biology. This appeal to scientific authority often seems to misrepresent what evolutionary theory predicts: that which evolves is not necessarily that which is healthy. McKenna’s theoretical stance is, of course, more nuanced than a simple equation of the natural with the good, and I am perhaps guilty of using him as a straw-man by taking published statements out of context. McKenna and colleagues have done valuable research on how mothers and infants interact while sleeping together and on variation among mother– infant dyads. Their proposal that co-sleeping and night-nursing have important benefits for infants is a reasonable hypothesis and deserves investigation. Evolutionary theory should be informed by observation, but observations are interpreted in terms of implicit or explicit theoretical models and may be misinterpreted if models are faulty. A single observation is often interpreted differently by different models. Hinde [2], for example, cautions against interpreting night-nursing as a tactic to increase interbirth intervals (IBIs) because marmoset mothers are most frequently woken by their infants around the time that mothers undergo post-partum ovulation [3]. Therefore, she reasons, night-nursing does not inhibit ovulation. However, maximal waking as mothers return to fertility is precisely what would be expected from a model of parent–offspring conflict. The observation does not discriminate between models of parent–offspring harmony or conflict. The return to fertility of tamarin mothers is delayed by more intense suckling [4]. target author’s response 57