Intelligent critiques of evolutionary theory are extremely rare, but they do exist. Probably the best of the lot is philosopher David Stove. Stove has zero sympathy for religion; his complaint about evolutionary theory is that it makes false predictions.

One of his best challenges (from his book Against the Idols of the Age): Parents share 50% of their genes with their kids. As you would expect, then, parents love their kids and make all sorts of sacrifices for them. But there’s a little problem here: Full siblings ALSO share 50% of their genes, and yet they notoriously can’t stop punching each other. If evolution were the whole story, says Stove, siblings would be as kind to each other as parents are to their kids.

I call this the “Sibling Paradox.” And while it’s a thought-provoking example, a little economics goes a long way to resolve it.

To clarify matters, suppose everyone has the same utility of wealth function, and, as usual, there is diminishing marginal utility. Now if two people had equal levels of wealth, and shared 50% of their genes, would they want to help each other? No. If we both have $1000, and I give you $100, then (a) Because of diminishing marginal utility of wealth, the gift helps you less than it hurts me, and (b) Because I discount your utility by 50%, I value the benefit to you at half the cost to myself. Surprisingly, then, two people who shared half their genes would still demonstrate no altruism toward each other as long as they were equally rich. Their mutual altruism is only latent.

What would it take to make people want to altruistically give away resources with no hope of repayment? Big differences in wealth. If we share half our genes, but you’re a pauper and I’ve got food to spare, I’ll be happy to help you out.

What does this have to do with the Sibling Paradox? Lots. Since siblings are normally roughly equally rich, it’s not surprising that they fight over toys. And since parents are normally much richer than their kids – who arrive in the world with nothing but what their parents give them – it’s not surprising that parents give and give.

Further predictions that seem right: Once kids become adults and are able to support themselves, the flow of parental assistance tends to dry up. Conversely, if siblings are far apart in age, the older one usually treats the younger one with a lot more generosity than if they were close in age.

Everyone seriously interested in evolution ought to read David Stove. He’s that good. But combined with a little economics, the Sibling Paradox gives evolutionary theory another chance to shine.