In this way, Aristotle's "friendship" coincides with what David Hume and Adam Smith called "sympathy"--any kind of "fellow feeling" among human beings. Charles Darwin adopted this idea and made "sympathy" one of the fundamental themes in his evolutionary account of moral and political order. More recently, biologists and psychologists have used the word "empathy" in a way that largely corresponds to what Hume, Smith, and Darwin would call "sympathy," or what Aristotle would call "friendship."
Running through all of this research is the idea that "friendship," "sympathy," or "empathy"--the psychological disposition that underlies social bonding--arises originally from the biological bond between parent and child, and particularly, mother and child.
Aristotle observes that human beings are not only political animals by nature but also household animals by nature. Indeed, the human coupling instinct is in some sense more natural than the political instinct, because human beings could exist in families even without living in political communities, as was the case throughout early human history when human beings lived in foraging groups of families. Moreover, the various forms of friendly feeling that unite human beings as individuals, as fellow citizens, and as members of the same species, radiate out from the natural affection between parents and offspring that human beings share with birds and other animals (1155a1-33, 1159a27-37, 1160b23-62a29). "Consequently, in the household are first found the origins and springs of friendship, of polity, and of justice" (EE, 1242b1-2).
Darwinian biologists have noticed that animals with the greatest cognitive capacities are often those with the longest periods of childhood dependence on adults. Aristotle agrees:
It would seem that nature wishes to provide for a sensation of attentive care for the offspring. In the lower animals, nature implants this only until birth; in others, there is care for the complete development of the offspring; and among the more intelligent animals [phronimotera], there is care for its upbringing. Among those who share in the greatest intelligence, there arises intimacy and friendship even towards the completely grown offspring, as among human beings and some quadrupeds. (GA, 753a8-14)
Through their love for their children as extensions of themselves, husbands and wives strengthen their marital bond because children are a common good, which is why childless marriages are more easily dissolved (1162a16-28). The parental affection of mothers is greater, however, than that of fathers, both because mothers must invest more effort in pregnancy and childbirth, and because they are more certain of their maternity than fathers are of their paternity (1161b16-29, 1166a1-9, 1168a20-27).
Beyond the bonds of kinship, unrelated individuals can develop friendly affection based on a reciprocal exchange of benefits. Rejecting any cosmological explanation of friendship as a force of attraction in physical nature, Aristotle argues that friendship must be a psychic relation among animals, "for there is friendship when like-mindedness [eunoia] is reciprocal" (1155b34). In the noblest friendships, people benefit others without expecting anything in return. But all or most people choose what is beneficial to themselves. In most cases, therefore, the recipient of a benefit is expected to return the equivalent of what he has received. Social conflict arises when people think this reciprocity has not been maintained (1162b22-65b37).
Not only personal friendships but also political communities are held together by a reciprocal proportionality of benefits (1132b33-33a5). People unrelated to one another can form associations based on calculations of mutual benefit. The political community arises from less associations to secure the common advantage of citizens for the whole of life (1159b25-62a34). Every community rests on some sense of friendship founded on the common advantage of its members. Although the strongest feeling of common advantage is among those who are biologically related, other bonds can arise as there is any reciprocal sense of shared needs.
As a consequence of his biological understanding of animal bonding, Aristotle sees the moral and political obligations of human beings as a series of concentric circles around the individual. Insofar as justice coincides with friendship, the claims of justice vary in proportion to the nearness of attachments (1155a16-29, 1159b25-60a8, 1165a14-36). One's obligations are stronger to closer relatives than to more distant ones, and stronger to close friends and fellow citizens than to strangers, although there is some friendly attachment to all members of one's species based on shared humanity (philanthropia).
As a biologist, Aristotle affirms the unity of humankind as one species, "simple and having no differentiation" (HA, 490b18). He believes there can be a kind of sympathy among animals of the same species, and this is especially true for human beings, so that "we praise those who love their fellow human beings" (1155a20-21). But the humanitarianism of human beings will always be difficult to cultivate and almost always weaker than their egoism, nepotism, and their patriotism. This explains the mistake of Plato's Socrates in proposing the community of wives and children and the communal ownership of property for the guardians in the Republic: it is unreasonable to ignore the natural love of oneself and one's own that makes it difficult for people to live together if they must share everything (Pol, 1262b22-25, 1263a41-b41). In fact, Socrates concedes that even in the best political community, the warriors would have to be taught that justice is doing good to friends and harm to enemies (Plato, Republic, 332a-3, 375a-e, 469b-d; Cleitophon, 410b).
Aristotle's biological claim that parental care for the young is the root from which all other social bonds grow finds support in Darwinian biology. "The feeling of pleasure from society," Darwin believed, "is probably an extension of the parental or filial affections, since the social instinct seems to be developed by the young remaining for a long time with the parents" (2004, 129).
This personal bonding between parents and their offspring distinguishes the vertebrates from the insects. Despite the functional similarities between insect and vertebrate societies, the crucial difference is that, in contrast to the impersonal character of insect societies, the vertebrates depend on personal recognition among members of a group. The efficiency of insect divisions of labor among castes was made possible by the evolutionary novelty of sterile castes, which means that the colony becomes the unit of natural selection. But vertebrates depend on individual reproduction. The cooperating castes among social insects are generally sterile, which lessens the genetic competition between cooperating individuals, which makes possible the evolution of self-sacrificial altruism. Among the social vertebrates, by contrast, genetic competition between cooperating individuals impedes the evolution of self-sacrificial altruism. As Edward Wilson has noted, the evolutionary path taken by the social vertebrates involves a trade-off, because it "enhances freedom on the part of the individual at the expense of efficiency on the part of society" (1971, 460). This explains whey the city described in Plato's Republic looks in some respects more like a bee hive than a community of human beings.
Modern ethologists have shown the importance of the mother-infant bond, especially for primates, as the root of all social bonding. Psychologists have noted the harmful, and sometimes fatal, effects of maternal deprivation on infants. We have learned much about the neural and hormonal changes during pregnancy, childbirth, and lactation that promote maternal behavior (as surveyed by Sarah Hrdy and Melvin Konner). Evolutionary psychologists have gathered evidence for the idea of Robert Trivers that, since females generally invest more in their offspring than do males, females tend to be more devoted to the raising of the young (Trivers 1985, 203-38).
On this, as on so many points, evolutionary psychologists are rediscovering what was originally discovered by Aristotle in his biological studies.
Some posts on the biology of friendship, sympathy, or empathy can be found here, here, and here.