My Brother's Keeper



Photo by pugphai at freedigitalphotos.net.

Relationships with siblings tend to be complicated. We love them, but they drive us nuts. We want to help them, but not if it takes too long or costs too much. These are common struggles because the psychology of these relationships is in our biological heritage.

Let’s play a thought experiment: Say a runaway trolley is heading down the tracks. Ahead of the trolley are five people tied to the tracks that are about to become mush (Don’t ask why they are tied up on the tracks – Just assume the evil boogeyman got them). You are in the distant train yard next to a lever. If you pull the lever, the trolley will switch to another set of tracks, sparing the lives of the five tied-up people. But… there is one person tied to the other set of tracks, so pulling the lever would also kill that one person. What do you do?



Photo by David Ingham at Wikimedia.

This is the classic scenario in a field of ethics called “trolleyology”. (Yes, this is a thing). If you are like 9 out of 10 people, you chose pretty quickly to kill the one person to save the five. Our utilitarian morals tend to value more lives over fewer.

But what if that one person is your daughter? Your mother? Your brother? The dilemma just became harder, didn’t it? Not because the one life is now inherently worth more than the five… but because that one life is worth more to you.

We are a social species that tends to live in family groups, as were our ancestors for over 100,000 generations. We also share more of our genes with our closest family members through shared inheritance. For example, we each share about half of our alleles (our particular version of a gene) with each of our biological parents. Our siblings with the same two parents have also inherited about half of each parent’s alleles, but not necessarily the same ones that we did. This means that we share about half of our alleles with our full-siblings too (and we share about a quarter of our alleles with our half-siblings). Following this same math, we share about a quarter of our alleles with our aunts and uncles and about an eighth of our alleles with our cousins. The more distant the relative, the fewer alleles we have in common.

Now let’s play another thought experiment: Say there is a population of animals where some individuals have lots of babies and raise their children to be good parents, and other individuals don’t have a lot of babies. An individual that has the “have lots of babies” and “be a good parent” alleles are going to share, on average, half of those alleles with each of their many children. They are also going to have more surviving children than most of the individuals that do not share those alleles. So, over many generations, more individuals in the population are going to have the alleles that make them have more babies and take care of them, right?

But there is another potential outcome to this thought experiment. Yes, we share about half of our alleles with our children, but we also share about half of our alleles with our full-siblings. Therefore, a “take care of your siblings” allele has the same probability of spreading through the population over generations as a “be a good parent” allele.

The genes that predispose our behaviors and decisions have been shaped by the generations of our ancestors that determined which of their relatives would have the support to raise more or fewer children… or none at all. In the 1960s, William Hamilton proposed a mathematical rule (called Hamilton’s Rule) for just such a decision:

rbB > rcC

where B is the number of individuals that survived because of you, rb is a measure of relatedness to those individuals, C is the number of individuals that died without your help, and rc is your relatedness to those individuals. This means that to pass on the most of our genes, we should factor in how many individuals and how closely-related they are to us when we decide who to help and who to sacrifice. 

For example, if you help to raise three of your siblings, each of which share ½ of their alleles with you and who would have died without you, but this means that you can’t take care of your own child that dies as a result, then this equation would be (½) X 3 > (½) X 1. This means that, mathematically, you should opt to raise your three siblings rather than your own child. J.B.S. Haldane, an influential geneticist, summed this decision process up when he was asked if he would give his life to save a drowning brother by responding “No, but I would to save two brothers or eight cousins”.

Inserting our close family members into the hypothetical trolley problem makes us squeamish because it puts the “take care of your family” alleles we have inherited in conflict with our utilitarian morals. We are literally the species we are today because we take care of our brothers and sisters as well as our children… even when they’re obnoxious.