The family is the smallest social structure that is reproductively complete. That is, individuals are not reproductively complete – it takes two to tango – and no other way of organizing the individuals who make up a family fully satisfies their survival and reproductive needs.

The Father

A fully-grown man, about 30 years of age, is self-sufficient in terms of survival. In fact, he runs a surplus: he can keep additional people alive if he wishes to. Now, 30 years old sounds a bit high, given that the age of majority in most countries is 18, but it is biologically correct1 2.

However, mere survival is short-sighted. A man, having reached maturity, understands he must produce an heir and raise him to an age of sufficient maturity to pass his assets and liabilities on to him. A man cannot achieve this alone; he needs exclusive access to a woman in order to produce a child he can be sure is his own.

The Mother

A sexually-mature, fertile woman is deficient in terms of survival. Without the direct or indirect protection of men, women simply don’t survive. However, reproduction is a non-issue for a woman: if she decides to reproduce indiscriminately, she can be pregnant by the end of the day. The natural bargain, then, is that the sexes exchange their surpluses, which coincide with the other’s deficiency.

The institution of marriage has always, until the recent era, codified this exchange: a woman offers exclusive access to her reproductive system to a man in exchange for his protection and providence. Of course, there are alteratives, such as the method of the prostitute: exchange non-exclusive reproduction with many men for a partial fulfillment of her needs.

However, of all the possible deals men and women can strike to supplement each other, only one offers reasonable assurance that the children were sired by the mother’s provider. While this assurance has been unreliable and frequently violated in the past, with today’s understanding of genetics, we are able to detect female infidelity with near certainty.

The Children

Children, I will not have to belabor, are wholly deficient. They are woefully underequipped to survive alone, and will have to survive for many years before reproduction even becomes a component of their lives.

Children in and of themselves have nothing to offer an adult; however, children are precious to their parents as carriers of their genetic legacy. As such, parents are inclined to assure the survival of their children, as they themselves will not be able to live indefinitely. In exchange for protection, children enable the parents’ bloodline to outlast them.

The Nuclear Family

Taking these parts together, they form the smallest reproductively self-sufficient grouping of human beings. Each party’s needs are fulfilled by another, and each party offers at least one other party a worthwhile benefit. Furthermore, children are maximally valued under this system that offers the father the greatest assurance that the children are his own.

While we have considered alternative arrangements between men and women, to the extent that they deviate from the women being exclusively available to one man, they reduce the assurance that the children are truly fathered by the provider, and thus undervalue children. These alternative arrangements have been practiced successfully in the past, but only where men did not understand the realities of reproduction: that a child has only one father, and that the first man to inseminate a fertile woman enjoys a very large advantage that the resulting child is his.

Given that we understand reproduction now better than we ever have, and we have known the key facts that make all alternatives to the nuclear family unworkable since biblical times, then, practically speaking, there are no alternatives. At the same time, since we now can genetically test for paternity, this arrangement has never been better.