The law of law’s leverage

Last week I posted on Owen Jones’s 2000 article Time-Shifted Rationality and the Law of Law’s Leverage: Behavioral Economics Meets Behavioral Biology and his argument that behavioural economics (and law) requires the theoretical backbone of evolutionary biology.

The second half of that article has a neat idea – what Jones calls the law of law’s leverage. The basic idea is that the effectiveness of laws will vary with the adaptiveness (in ancestral environments) of the behaviour the law is trying to change. Jones describes it as follows:

The law of law’s leverage predicts that less legal intervention will be necessary to shift a behavior in ways that tended to increase reproductive success in ancestral environments than will be necessary to shift behavior in ways that tended to decrease reproductive success in ancestral environments. Put another way, the slope of the demand curve for historically adaptive behavior that is now deemed to be socially (in some cases even individually) undesirable will be far steeper than the slope of the demand curve for behavior that was comparatively less adaptive in ancestral environments. Importantly, this relationship between the slopes will hold, even when the costs that an individual actually and foreseeably incurs in behaving in an historically adaptive way will exceed presently foreseeable benefits of such behavior

As an example:

Evolutionary analysis predicts that, and explains why, the slope of the demand curve for adulterous behavior is likely to be comparatively steep (as is the slope for most sexual behavior) and thus comparatively insensitive to the imposition of legal prohibitions (or other costs, such as effect on career). Evolutionary analysis also predicts that, and may help explain why, marriage, separation, divorce, and remarriage behavior will be less sensitive to legal changes than will be many other forms of behavior. Because, as we know, natural selection disfavors inbreeding among close relatives, evolutionary analysis also and separately predicts that it will be far less costly to discourage incest among parents and their natural children, and between siblings reared together, than among stepparents and stepchildren, and among stepchildren. Because we know that natural selection favors discriminative parental solicitude rather than indiscriminate parental solicitude (that is, it generally favors psychological mechanisms that bias resources toward offspring over non-offspring), we can explain and anticipate that the cost of reducing child abuse will be greater, per capita, for stepparent households than for non-stepparent households. Similarly, we can predict that men under court-order to provide child support payments for a child they know or suspect they did not father will be less likely to comply than will biological fathers.

Another nice example brings the point home strongly:

Take, for example, a hypothetical legal rule that required an adult, in a crisis situation involving both her children and the children of others, to save children in order of their ranked intelligence (or any other desirable characteristic), irrespective of her own relatedness to each. We know that such a legal rule would be absurd. But why? It is not because the rule would lead to inefficient outcomes. To the contrary, the outcome might increase social wealth compared to the alternative.

It is not enough to say that powerful social norms would generate irresistible emotions in the woman to save her own children, because it so happens that we would expect the same behavior from parents all over the world, regardless of the many vicissitudes of culture. We know the rule would be absurd because we intuitively sense that the preference to save one’s own child would be insensitive to variations in legal costs we might impose in an effort to shift the behavior—all over the planet, in every human culture. The theoretical basis for that sense of relative inelasticity of the demand for certain behaviors, in certain contexts, is not simply acculturation alone, but the law of law’s leverage, as derived from the effects of evolution on human behavior-biasing psychological predispositions.

The concept of the law of law’s leverage reminded me of a section in Steven Pinker’s The Blank Slate: The Modern Denial of Human Nature, when he pointed out that evidence of a biological basis to crime could be used both by advocates of stronger punishment and advocates of weaker punishment. Evidence of a biological disposition could be argued to reduce culpability. Alternatively, a strong disposition needs to be countered by even stronger incentives.

As such, the law of law’s leverage may lead us to look at a steep demand curve for a particular behaviour and acknowledge that we can’t change it. Or, we may want to massively change the price.


  1. A very eloquent restatement of some very basic (I mean, entry level basic) evolutionary biology in terms that economists are comfortable with … the stuff is starting to penetrate other disciplines, I guess… but hardly anything like a new law or law’s law or whatever rhetorical claim it is meant to support…just basic biology applied to the humanly created world of econs

    Great stuff as always Jason. A real pleasure each time…

    Best to all,


  2. Demographers studying fertility patterns in Western Europe, where the data is very good, find that extramarital fertility is so low as to be negligible. One possibility is that laws–more generally social institutions–rather easily bends us away from a naturally adulterous behavior, or that natural human behavior is not adulterous. Bernard Chapais argues that rearing human infants requires a cooperative breeding system in which it is highly advantageous to recruit fathers and paternal kin to raise an infant. If so high paternity certainty is a root of human sociality and most of us are not naturally adulterous. Supporting this idea, Robert Martin produces fairly compelling evidence that human reproductive biology is not consistent with a highly promiscuous pattern of mating. Owen Jones and you are a victims of of a pop version of evolutionary psychology that is not based on a serious examination of the evidence on human evolution.

    Pete Richerson

    Chapais, B. (2008). Primeval Kinship: How Pair-Bonding Gave Birth to Human Society. Cambridge MA: Harvard University Press.

    Coale, A. J., & Watkins, S. C. (1986). The Decline of Fertility in Europe. Princeton, N.J.: Princeton University Press.

    Larmuseau, M. H. D., Vanoverbeke, J., Van Geystelen, A., Defraene, G., Vanderheyden, N., Matthys, K., . . . Decorte, R. (2013). Low historical rates of cuckoldry in a Western European human population traced by Y-chromosome and genealogical data (Vol. 280).

    Martin, R. D. (2013). How We Do It :The Evolution and Future of Human Reproduction. New York: Basic Books.

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