Sometimes completely random questions pop into my head. Yesterday my completely random question was, why has it been common throughout history that inheritances go to first born male children (if that is in fact true across most cultures)? Actually, it wasn’t a completely random question, the seed of the idea was planted when I was talking to Jo about “little r” for mosquito populations. I had never really thought about it before, but the strategy of bequeathing wealth to an eldest male could make sense from an evolutionary point of view. I’m sure many people with evolutionary backgrounds have thought about this before (and probably studied it in substantial detail). Boiled down, there are two patterns that need to be explained, 1) choosing to leave resources to male over female children, and 2) choosing to leave resources to elder over younger children.
I know that the first choice has been discussed in animal populations to explain biased sex ratios within families. When one sex has more variable realized fitness than the other, high resource parents should try to have the more variable sex because their offspring will have enough resources to develop into high quality individuals that can be highly competitive for mates. Conversely, low resource parents should try to have the less variable sex. Extending this theory to resource investment among siblings, parents should biasedly invest resources into the siblings that are of the more variable sex. In human populations, male reproductive fitness is (or at least historically was) more variable than female reproductive success, and so from an evolutionary point of view, parents should have been more willing to leave their inheritances to males than females. Fitness in modern society is governed by very different factors from that in the past, and so whether this conclusion still follows today is unclear.
The logic for the second choice was a little less obvious to me, until I started thinking about fitness in terms of “little r”. For anyone unfamiliar with my use of the term “little r”, I am referring to the intrinsic rate of growth of a population (e.g. dN/dt = rN). Number of offspring births is obviously important to this value, but perhaps less obvious is that the timing of these births is also very important in a growing population. An individual who has three births spread out over 20 years, is much less fit than an individual who has three births spread out over 10 years, because the latter individual has a shorter generation time. First born children on average are more important to fitness than latter born children, and so evolutionary theory should favor leaving inheritances to elder over young children. Add to that the fact that younger children have more risk of dying before reaching reproductive age, and the effect only gets stronger.
I wonder if there are any other species that have inheritance-like events. I would be super interested in learning whether this logic holds up for other species.