Cristina Moya et al.
Inter-generational conflicts over reproductive decisions: A cross-cultural examination of parental presence effects on age at first birth
Cristina Moya (London School of Hygiene and Tropical Medicine (LSHTM) Rebecca Sear(LSHTM), Borgerhoff Mulder(UC Davis), Colleran(UCL), Costa(LSE), Gervais(UCSB), Gibson(Bristol), Goodman(LSHTM), Gurven(UCSB), Henrich(UBC), Hooper(Emory), Kaplan(UNM), Kline(UCLA), Koster(U of Cinncinati), Koupil(CHESS), Kramer(U of Utah), Leonetti(U of Washington), Mattison (U of Auckland), Scelza(UCLA), Shenk(U of Missouri), Snopkowski(LSHTM), Stieglitz(UNM), von Rueden(U of Richmond), Ziker(Boise State), Bowles(SFI)
The presented paper has been removed at the request of the authors
Testing comments.
I’d like to thank the discussant for the detailed comments as they will help us make the arguments and data analysis strategy clearer. Below are some thoughts in response.
I completely agree that the background section should better frame the sex differences and cultural differences that we discuss later on and am working on that at the moment.
I’d like to pick up on one of your points about the discussion being one-sided in looking at the pre-reproductive usefulness of adolescents as it illustrates the different (and admittedly often naive) default assumptions of evolutionary demographers. In terms of individual fitness maximization (that is leaving the most descendants in future populations relative to others in the population), evolutionists argue that starting reproducing is generally beneficial all else equal. Life history theorists suggest that delaying reproductive onset may be selected for in contexts where there are long-term fitness benefits to growth or educational investments that are reaped over the life course (e.g. someone who starts reproducing at 18 is in better condition than someone who starts reproducing at 14 and thus leaves 8 instead of 6 offspring). According to this world view what has to be explained is why delay at all, especially given that delays in many of the populations we study go well beyond the age of reproductive maturity, and have been shown in some cases to not maximize fitness.
I completely agree with you that there are financial and status costs associated with reproducing in many societies, and that these guide reproductive decision making, despite the negative fitness consequences of privileging status in some of these. I do make this argument briefly in the discussion where I say that particularly in post-industrial societies the status benefits to delaying first births might overwhelm fitness maximizing behavior, but I should elaborate on it further, and suspect that it may be important in pre-transition societies as well.
Along these lines, the asymmetric costs to reproducing for women and men should matter only insofar as these are social costs, but should not differentiate female and male’s strategies if we model them as fitness maximizers. From an evolutionary theoretical perspective the fact that you pay more costs than your partner for reproduction doesn’t necessarily mean that you will have divergent reproductive interests (e.g. the fitness benefits to reproduction may outweigh the costs even if our costs are different). Now I agree that women and men experience different social, health, psychic costs to reproduction and that these likely (sometimes maladaptively) affect their decisions to reproduce, but these should be relatively culture-specific and it would be hard to get data to code the different societies in our sample appropriately. Perhaps of interest, some evolutionary theorists have suggested that precisely because of these psychic and health costs that deter women from reproducing, kin serve a pro-natal function of encouraging reproduction for their own long-term inclusive fitness benefits (i.e. they can increase the likelihood of their own genes in future generations by improving genetically related individual’s reproductive success).
Barkow & Burley. 1980 Evolutionary Biology, Human Fertility, and the Demographic Transition. Ethology and Sociobiology 1:163-180.
Other points
1. You are right the mean effects of mothers and fathers seem to be quite similar, although their distributions across societies varies, and their effects are moderated by different group-level variables. In our very simple inter-generational conflict model the absence of either parent should be enough to decrease parental delays to an adolescents’ first birth. This is because the adolescent would benefit more (in re: fitness) from having her own child than helping raise half siblings. In other words, the conflict between an adolescent and a parent with a new spouse is symmetric.
2. I should clarify that I mean the world institution in the loosest sense of a socially learned cultural belief or pattern that has the potential to constrain or influence individual’s decisions within a society.
3. The intrinsic nature of some of these variables is definitely troubling. While you suggest that ambi/matri/patri locality are intrinsically linked to our parental availability measures, we tried to avoid this as much as possible by using parent’s vital status as the measure when possible. In that case, we assume that one’s mother is as likely to be alive at time of interview, regardless of the society’s post-marital residence norm (all else equal).
4. Thank you for the suggestion to look at a society’s gender preferences as a potential moderator. This could definitely have an effect, as might many other cultural differences we have yet to test (e.g. polygyny, bridewealth/dowry).