Bryan Caplan  

Resolving the Sibling Paradox

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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.

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The author at Ashish's Niti in a related article titled Darwinism, anyone? writes:
    Link: EconLog, Resolving the Sibling Paradox, Bryan Caplan: Library of Economics and Liberty. 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 sy... [Tracked on July 24, 2005 1:15 AM]
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Matt McIntosh writes:

Oh I don't know, I wasn't terribly impressed with Stove's Scientific Irrationalism, particularly the way smeared Popper, Lakatos, Kuhn and Feyerabend together into some sort of four-headed monster. He makes a few good points, but a lot of others are just so much intuitionist table-pounding.

Assuming you've accurately described Stove's criticism here, I can see he's at about the same level of quality when criticizing Trivers as he is of Popper. The argument he attacks sounds pretty clearly like a strawman. You yourself give the perfectly sensible answer, and this is pretty much exactly what any competent sociobiologist would say as well. The economics of competition for scarce resources is an integral part of Darwinism and always has been.

Patri Friedman writes:

Yeah, the simple way I'd put it is that siblings main source of resources is their parents, so they compete over that source. Parents don't compete with their kids for resources! (And when they do, on occasion, parents often act selfishly).

I've always thought that sibling behavior fit perfectly with evolutionary biology. Fierce in-fighting over parental resources, but protectiveness of family w.r.t. outsiders

Will Wilkinson writes:

The answer is in Robert Trivers. In a nuthsell: Parental resources are scarce; kids have to compete for them.

deb writes:

Let me get this straight. You believe that:

a. the empirical observation that siblings fight is evidence against Darwin's theory of evolution


b. while this paradox cannot be resolved within evolutionary theory, it can be resolved by economics.

Do you really believe that if you presented the "sibling paradox" to Dawkins or Dennett, they'd say "Wow. I never thought of that. That IS a problem for evolutionary biology. Thank you economics for saving us."

Do you really have such a fragile understanding of evolution that you can't imagine a response like Will Wilk's or the dozens of other obvious evolutionary explanations for sibling rivalry (e.g., relatively harmless way to practice competitive behavior to increase skill in real world competitive situations).

Your ignorance of evolution is appalling but not surprising.

Robert Schwartz writes:

I am not a fan of "Evolutionary Psychology." I have never seen any report of its results that did not strike me as ad hockery and just so stories. This one is no different.

The first thing that must be established and is not, is whether the studied behavior is biological in origin. I know that sibling rivalry is the oldest story in the world (Jacob and Esau, Joseph and his brothers), but I do not know if it is an invariable, biologically driven aspect of human behavior.

Further, I do not know that sibling quarrels have a biological impact. In their hour of need, Joseph saved his brothers from starvation. Similarly, it is often the case that siblings will unite against an outside threat instead of allying with outsiders to advance their interests against their siblings.

Incidentially, Brian, I know that quarelling children can send a parent up the wall. I have found it best to treat the quarelling as the problem not the ostensible basis of the quarrel. Often they are only quarreling because they are tired and cranky. Ignore cries of unfairness. Send them all to their rooms.

Peter Gallagher writes:

Dear Brian,

This lovely, cheeky essay on the 'higher darwinism' is one of many from the contrarian's delight, David Stove. He was a teacher and tutor of mine for one year only at the Sydney University Dept of (internecine) Philosophy in the early 1970s, but he was one of those teachers whose reputations grows with us as we get older. He died in 1994 but a memorial to him is here ( with links to several of his works on-line. If you agree that he punctures some of the sillier inflations of Darwinism, try his attack on the 'cult' of Plato.

Best wishes,


Gregory Kettler writes:

Out of curiosity, Bryan, are you an only child?

Any time I saw my brother in pain, it was devastating. And yet we would squabble for hours over trivial things. I think you underestimate a child's ability to differentiate between what's important for survival and what's fair ground for an entertaining argument.

Bernard Yomtov writes:

You don't need to assume diminishing marginal utility of wealth. Your argument works just fine without it since the siblings discount each others' utility.

So your analysis just reduces to the fact that, as Patri and Will point out, siblings are competing for resources. I haven't read Stove, but if that's "one of his best challenges" I think I'll skip him.

Steve Reuland writes:

Stove's critiques of evolution, if you can call them critiques, are an outright travesty. Trivers successfully explained parent-offspring conflict back in the 70s, and was in fact able to make a number of specific predictions that turned out to be true. (See here and here for example). One of these days, Stove might actually make an effort to understand what evolutionary biologists have been doing all these years.

Stove is "that good"? Well, obviously the good man has never heard of chaos theory, self-referential systems, autopoiesis, and neuronal networks. While he touches on some thought-provoking issues with the choice of Darwinist dogmas he challenges, his own argumentation remains depressingly week. To counter scientific hypothesis (and even evidence) with the call for trusting "common sense" is the same argumentation that was used to maintain that the earth is flat and the center of the universe. For a philosopher, this level of argumentation is just poor.

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