There Are Only Two Levers on Human Behaviour, and We Can Pull One

Behavioural genetics has settled a question that used to be a matter of temperament. Human behaviour is roughly half heritable. Across traits, across countries, across decades of twin and adoption studies, the split between genes and everything else comes out near the middle. I have written about how uncomfortable that finding is and why the discomfort is not an argument. Here I want to draw the practical conclusion, because I think it is more important than the finding itself.

If behaviour has two sources, there are exactly two levers for changing it. One is the genome. The other is the environment, which for an adult human mostly means the society he lives in. There is no third. Anyone who wants people to behave differently — less violently, more cooperatively, with longer horizons — is going to pull one of these two, whether they know it or not.

The Genetic Lever

The first lever is, for now, barely a lever at all.

We do not know which genes do what for almost any behavioural trait. The traits are massively polygenic: thousands of variants each contributing a sliver, interacting with each other and with the environment in ways nobody can currently model. Predicting a person's temperament from their genome is possible only crudely, and altering it is not possible at all.

Suppose that changes. Suppose in a century we can edit the genome for temperament as we now edit it for single-gene disease. Would we want to?

Consider the trait that causes most of the trouble: selfishness, the tendency to weigh one's own interest above the group's. It is heritable. It is also, very plausibly, load-bearing. It is entangled with ambition, with the drive to protect one's children, with the refusal to be exploited, with most of what makes a person a person rather than a component. An engineered reduction in selfishness would not produce a better human. It would produce a different species, and we have no idea what else it would take with it.

So even in the optimistic case the genetic lever is one we should be reluctant to touch, and in the actual case it does not exist. That leaves one.

The Environmental Lever

The second lever is the one every society has always been pulling, mostly without admitting it.

Environment, for an adult, is not weather and diet. It is the incentives around him: what is rewarded, what is punished, what is visible, who is watching, what the people near him are doing, what the institutions he depends on require of him. Change those and behaviour changes — not because the person changed, but because the same person is now responding to different conditions.

We know this works because we watch it work. The same population, with the same genes, behaves one way under a regime that punishes initiative and another way a decade later under one that rewards it. Crime rates move with policing and prosperity far faster than any gene could move. Cooperation appears and disappears with the structures that make it pay.

The heritability figure does not contradict this. Heritability measures the share of variation between people attributable to genes within a given environment. It says nothing about how far the whole population would shift if the environment changed, and the answer to that is: a great deal. Height is more heritable than most behaviour, and average height moved several inches in a century through nutrition alone.

Why This Is a Political Conclusion

Put the two halves together and the argument closes.

Half of behaviour is genes we cannot and probably should not alter. The other half is environment, and the environment that matters is the design of society: its laws, its institutions, its distribution of power, the way it decides things. The design of society is therefore the only instrument we possess for improving how human beings behave. Not one instrument among several. The only one.

That reframes what politics is for. Politics is not, on this view, the administration of a fixed human nature, working around defects that cannot be fixed. It is the operation of the single lever that can change the behaviour of a species that will not change its genes.

What Follows for the Design

If society is the lever, then the question of how society makes decisions is not one political question among many. It is the question, because it determines who gets to set the environment everybody else responds to.

Under a hierarchy, a small group sets the conditions for the rest and, naturally, sets them to suit itself. The environment the majority lives in is then designed by a minority for the minority's benefit, and the behaviour it produces in the majority — resentment, defection, short horizons — is exactly what you would predict from conditions designed by someone else.

A society that sets its own conditions has at least the possibility of setting them well. That is the case for spreading the decision, stated in terms of behavioural genetics rather than fairness: the environmental lever should be held by the people whose environment it is.

The genes are given. The society is chosen. We have spent a great deal of effort arguing about the half we cannot change and remarkably little on designing the half we can.