The Nature of Parent-Offspring Conflict over Mating: from Differences in Genetic Relatedness to Disagreement over Mate Choice

Abstract

Parents and children are genetically related, but they are not genetically identical. This difference leads to diverging interests and ensuing conflict. There are two, not mutual exclusive, hypotheses that propose to account for how the divergence in genetic interests leads to conflict over mating, namely the compromises in traits and the evolutionary trade-offs. The present paper attempts to demonstrate that the compromises in trait hypothesis can account for this conflict, but the evolutionary trade-off hypothesis cannot. It also aims to combine insights from the two hypotheses in order to provide a better account of the nature of parent-offspring conflict over mating. In the proposed synthesis, compromises in desirable traits lead to parent-offspring conflict over mating, with evolutionary trade-offs regulating the degree of this conflict depending on the sex of the child exercising mate choice and the local conditions.

Appendices

Appendix 1

(a) The degree of the fitness benefits that individuals derive from a mate’s good genetic quality.

(B) The fitness losses that parents suffer from their children’s mate choices.

(B _child) The fitness benefit that individuals can derive from a mate.

(B _parent) The fitness benefit that individuals can derive from an in-law.

(b) The fitness benefit that individuals derive from a mate’s good family background.

(c) The fitness benefit that individuals derive from an in-law’s good genetic quality.

(d) The fitness benefit that individuals derive from an in-law’s good family background.

(F) The mate points that children or their parents allocate to good family background.

(G) The mate points that children or their parents allocate to good genetic quality.

(V) An individual’s mate value.

Appendix 2

In Apostolou (2011) study, children and their parents were given three different budgets of mate points to allocate in eight desirable traits, including good looks and good family background, in a prospective mate and daughter- and son-in-law, respectively. Assuming that good looks constitute a proxy of good genetic quality and by making assumptions about the fitness contributions of genetic quality and good family background, we can employ Eq. (3) in order to estimate the fitness cost that children’s allocation inflicts to parents. More specifically, let us assume that good genetic quality is twice as beneficial to children as it is to their parents, while good family background is half as beneficial to children as to parents so that a = 1, b = .5, c = 0.5, and d = 1. Substituting these values in Eq. (3)

$$ \left(a\hbox{-} c\right)G + \left(b\hbox{-} d\right)F = 0.5G\ \hbox{-}\ 0.5F $$

Now, we can take the actual allocations that children made in the Apostolou (2011) study. As an example, we are going to employ the allocations that children made for a prospective spouse in the medium budget (we chose this budget as more representative of an average mate seeker). Here, children allocated an average of 4.71 mate points to good looks and 3.08 mate points to good family background. Substituting these values to the formula above:

$$ 0.5*(4.71)\ \hbox{-}\ 0.5*(3.08) = 0.815 $$

The 0.815 represents the fitness cost to parents due to the allocations that their children chose to make on these traits. Further work that will estimate the contributions that each trait makes to the fitness of each party (i.e., a, b, c, and d) will enable more precise calculations.

Finally, please note that the Apostolou (2011) study reports the allocations in terms of percentages and not in terms of mate points. In order to calculate the mate points used here, the reported percentages were multiplied with the specific budget. So, for instance, if for a budget of 100 mate points, men had allocated 10 % of their budget to good family background, this translates into 10 mate points (100* 0.10).

Appendix 3

The Apostolou (2011) study gave parents and children three different budgets of mate points to allocate in different traits. The low budget had few mate points depicting a mate with low mate value, the moderate budget had more points depicting an individual of moderate mate value, and the high budget had even more points, depicting an individual of high mate value. Accordingly, this design can enable us to examine the hypothesis that the cost inflicted to parents is reverse U-shaped. More specifically, for each budget, we estimated the fitness cost inflicted to parents by the allocations of daughters and sons using Eq. (3). We have made similar assumptions as in Appendix 2 about the fitness contributions of each trait, and we assumed also that good looks constitute a proxy of genetic quality.

Accordingly, children in a low budget allocated an average of 2.42 mate points to good looks and 1.45 mate points to good family background so that substituting in the Eq. (3), we have the following:

$$ 0.5*(2.42)\ \hbox{-}\ 0.5*(1.45) = 0.485 $$

In the medium budget, they allocated 4.71 mate points to good looks and 3.08 mate points to good family background so that

$$ 0.5*(4.71)\ \hbox{-}\ 0.5*(3.08) = 0.815 $$

In the high budget, they allocated 6.28 mate points to good looks and 5.13 mate points to good family background so that

$$ 0.5*(6.28)\ \hbox{-}\ 0.5*(5.13) = 0.575 $$

We can see that the results of these calculations fit the prediction that the fitness cost inflicted to parents is a reverse U-shaped function of children’s mate value: As we move from low to medium budget, the fitness cost increases, whereas as we move from medium budget to high budget, the cost decreases.

One obvious limitation of the above calculations is that we do not have the actual estimates of the fitness contributions of the genetic quality and of the good family background. Another limitation is that the budget allocation study has only three different kinds of budgets (i.e., low, medium, and high); thus, we cannot get a more accurate picture of how the fitness cost changes with mate value.

Appendix 4

In order to examine whether trade-offs result in more conflict between parents and sons than between parents and daughters over genetic quality, we employed data from the Apostolou (2011) study. Similarly to Appendix 2, we assumed that good looks constitute a proxy of genetic quality, and we made also the same assumptions about the fitness contribution of each trait.

In the presence of trade-offs, daughters and their parents have to discount the loss in parental investment coming from the good genetic quality. In different words, the fitness contribution that the (G) makes is lower when trade-offs are present. Let us assume that trade-offs reduce the fitness contribution of (G) by 10 % so that a = 0.9 and c = 0.45. Accordingly, the fitness cost to parents from the mate choices of their daughters will take the following form:

$$ 0.45(G)\ \hbox{-}\ 0.5(F) $$

Furthermore, since a _sons > a _daughters, we would expect that sons would allocate more mate points to the good genetic quality than daughters. This is consistent with the evidence from Apostolou (2011) budget allocation study: In each budget level, sons allocated more mate points to good looks than daughters. We estimate the relative fitness costs to parents for the medium budget. More specifically, daughters allocated 3.88 mate points to good looks and 3.1 mate points to good family background so that

$$ 0.45*(3.88)\ \hbox{-}\ 0.5*(3.1) = 0.196 $$

On the other hand, sons allocated 5.53 mate points to good looks and 3.05 mate points to good family background so that

$$ 0.5*(5.53)\ \hbox{-}\ 0.5*(3.05)=1.24 $$

As 1.24 > 0.196, we can see that the fitness cost that the parents suffer is higher in the case of sons than in the case of daughters. It has to be said, however, that good looks constitute an imperfect measure of good genetic quality, as they also convey information about other traits of interest such as age (Gottschall 2007). Thus, men in this study may allocate more points to good looks also because they ascribe more value to youth in a mate than women and interpret good looks as indicating younger age.