The idea of human nature refers to the distinctive cluster of biologically inherited physical, behavioral, and psychological traits that characterize the natural human condition. Descriptions of some trait or other as part of human nature are often controversial, for three reasons: (1) there are disputes over what conditions need to be satisfied for something to count as “natural”; (2) claims about human nature are typically made without sufficient understanding of the hereditary basis of the traits in question, especially in the case of complex behavioral and psychological traits; (3) the identification of some human state as “natural” is seen as having ethical, social, and even legal implications. Eugenics can be understood as an attempt to induce changes in the cluster of traits that characterize human nature, promoting some traits and extinguishing others from the human population — just as the goal of farming and animal breeding is to generate a strain of organisms that reliably reproduces a certain cluster of desirable traits.

Naturalness
Physical, behavioral, and psychological traits take on different forms or states in different people. Some times these differences are enormous — for example, people differ with respect to their number of legs — but usually they are small, so small that we need measuring devices to detect them. One way to think of the naturalness of a physical, behavioral, or psychological state is by comparing it to the range of variation observed within the human population. Although there is some variation in the human population with respect to leg number, the range of variation is very narrow. Moreover, the state of having two legs occurs with a much higher frequency than any other state in that range. On one conception of naturalness, having two legs is a natural human state because it occurs with sufficient frequency. Having more or less than two legs is an unnatural human state because it is extraordinarily uncommon.

In principle, this way of understanding natural human states can be applied to behavioral and psychological traits. Just as we would conclude that having two legs is natural for humans because it is so common, we can sort natural from unnatural behavior by observing the relative frequencies with which each behavior occurs. Alas, humans exhibit an extensive range of behavioral variation. As the range of variation expands, inferences concerning the naturalness of a given behavioral state become more and more difficult to justify. Because so many people act and think in so many different ways, it is difficult to identify particular behaviors that achieve a relative frequency that is high enough to allow us to infer naturalness with the same degree of confidence as having two legs provides. This problem persists even when we subdivide the human population into smaller classes, such as man, woman, child, etc. In response to this basic fact about human behavioral and psychological variation, many have argued that there is no such thing as human nature, because there is no cluster of behavioral and psychological traits that occurs with sufficient frequency to be considered “normal” from a statistical standpoint (Hull 1986).

Statistical frequency is not the only method by which to approach the distinction between natural and unnatural human behavior and psychology. In some instances, what has been identified as the “natural” human state turns out to be nothing more than what is desirable to the person or group engaged in making the distinction, and what is “unnatural” turns out to be merely what is undesirable from that group’s perspective. It is not plausible to think that every identification of a human state as “natural” works this way, but history suggests that the practice is common enough to proceed with caution. Many societies (e.g Nazi Germany) have held marriages between individuals of different racial or ethnic groups (see miscegenation) to be unnatural for reasons that are now known to have been rooted exclusively in the subjective preferences of some group or other. Historically, the classification of certain behavioral or psychological conditions as pathological has also been susceptible to the intrusion of personal opinions concerning which sorts of people are desirable and which are undesirable.

Nature vs Nurture
The theory and practice of eugenics is founded on the presumption that the range of variation in human behavior and psychology can be manipulated by selective breeding in the manner of farming and animal breeding. In order for such a program to work, differences in the behaviors targeted for selective manipulation would have to be caused by genetic differences. In addition, those genetic differences would have to be hereditary. For example, were we to target jealousy -- an alleged aspect of human nature (Buss 2000) -- for elimination from the population through selective human breeding, our success would require that differences between individuals with respect to their propensity toward jealousy be in part attributable to genetic differences, and that those genetic differences be passed down from parent to child. Because the genetic bases for most human physical, psychological, and behavioral characteristics are unknown, it is plausible that many of the aspects of the human condition normally attributed to an intrinsic human nature are overwhelmingly, or at least significantly, the product of environmental influence on human development, psychology, or behavior. This, in turn, makes it difficult to know for any particular characteristic whether it satisfies the conditions required to manipulate its frequency through selective breeding.

Ethical Dimensions of Human Nature
One reason for the controversy surrounding assignments of some behavior as “natural” is the fact that perceptions of a trait’s naturalness affect our moral assessment of that trait. Historically, these moral assessments have frequently had legal or policy implications. Violent actions that are considered natural behavioral responses — for example, so-called “crimes of passion” — are often treated more leniently by the justice system than actions that do not enjoy “natural” status.

-Chris Haufe

• Buss, David (2000). The Dangerous Passion: Why Jealousy Is as Necessary as Love and Sex. New York: Free Press.

• Hull, David L. (1986). “On Human Nature.” PSA: Proceedings of the Biennial Meeting of the Philosophy of Science Association Vol. 1986, Volume Two: Symposia and Invited Papers (1986), pp. 3-13.

• Downes and Machery (2013), eds. Arguing about Human Nature: Contemporary Debates. New York: Routledge.

• Kronfeldner, Maria, Neil Roughley, and Georg Toepfer. 2014. “Recent Work on Human Nature: Beyond Traditional Essences.” Philosophy Compass 9 (9): 642–52.