Human parental age difference and offspring count: and we still do not know what men or women want.

Evolutionary biology Comment Human parental age difference and offspring count: and we still do not know what men or women want Two recent studies (Fieder & Huber 2007; Helle et al. 2008) present data on optimal age differences of human couples and show a hump-shaped relationship between parental age difference and lifetime offspring count. Boyko (2008) thereafter correctly pointed out that sex-specific locations of the peak offspring count are a logical impossibility (except for sampling error, see also Fieder et al. 2008a). Boyko's contribution is a welcome reminder that sexual selection studies must take into account the Fisher condition: the fact that male and female reproductive success is logically intertwined because each offspring has precisely one genetic father and one genetic mother. This simple statement is surprisingly often overlooked. Corrective action is frequently needed because a failure to take the Fisher condition into account can dramatically change evolutionary predictions (see Kokko et al. (2006) and references therein). Despite the laudable clarity of Boyko's short letter, its take-home message (expressed in the title as 'Optimal age difference cannot differ between monog-amous males and females') may lead to the impression that the interests of men and women must logically coincide when forming monogamous partnerships. Such an interpretation is mistaken, as is the original assertion that the effects of parental age differences allow inferences about best male and female options separately (Fieder & Huber 2007; Fieder et al. 2008a,b). As a thought experiment, consider a case where a pair's offspring count (lifetime reproductive success, LRS), after lifelong monogamy, equals LRS Z M x 2a x 1K x 2a x C y 2a y 1K y 2a y : ð1Þ Here, x and y denote female and male age (respectively) at the time of pairing; a x and a y indicate the ages at pairing that lead to the largest contribution to pair fitness; and M scales the LRS towards realistic values. This is a toy model, intentionally chosen so that male and female ages contribute independently to pair fitness. This makes it particularly easy to state the optimal mate choice rule. For a female of any age, the best choice is a mate of age a y , likewise, for a male of any age it is ideal to find a female of age a x. Figure 1a shows an example with a x Z20, a y Z30, together with the arbitrary assumption …