In 1960, most Americans held the traditional Christian belief, shared by Muslims and Jews, that premarital sex is wrong. Since then, public opinion has changed, and most people now think that premarital sex is okay, even healthy and good.

Our grandparents were right, and we are wrong. To see this today, we can add to their accumulated wisdom arguments that draw some premises from evolutionary psychology, the school of thought that explains human instincts as a set of strategies for the survival and propagation of our “selfish genes.” It has often been noticed that evolutionary psychology confirms many gender stereotypes; it turns out that it can also, when combined with some commonsense ethics, support a new defense of traditional moral views on sex.

Evolutionary psychology shows why casual, no-strings-attached sex does not come naturally to people and why they can’t, ultimately, be satisfied with it. Men are eager for it and use women for pleasure when they can get away with it, but women are naturally choosy and coy. They prefer sex in the context of a committed relationship. Premarital sex is messy and troublesome because of inherent conflicts of interest between men’s and women’s selfish genes. In this respect, it contrasts sharply with marriage, which creates a harmony of genetic interests.

Instincts, Ethics, and the Selfish Genes

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Richard Dawkins coined the phrase “selfish genes” to elucidate how evolution can explain altruistic behavior. Though the altruist may not serve his own interests, he serves the “interests” of his genes.

Humans often feel instinctive impulses to help others, but most of these arise from the strategies of our genes to help themselves. The simplest example of this is the instinctive altruism of parents toward their children. Parents make sacrifices, but their genes live on. Selfish genes influence us at a sub-rational level, feeding us desires, impulses, reflexes, and preoccupations—in a word, instincts.

Ethically, these instincts are not inherently either good or bad. C.S. Lewis, in Mere Christianity, explained it best, when he compared instincts to keys on a piano, and ethics to the pianist. No piano key is, in itself, a right or a wrong note, but each is a right or wrong note at a given moment in a piece of music. Likewise, our instincts are all good at certain moments in life, bad at others. The task of ethics is not to obey or to suppress instincts indiscriminately, but to govern them reasonably. Reason—a faculty belonging to the soul and irreducible to genes, molecules, or material forces—discerns the ends for which we should make use of our instincts.

Evolutionary psychology elucidates not human nature as a whole, but the part of human nature that Paul called “the flesh.” As a result of the Fall, humans have lived for hundreds or thousands of generations in conditions of competition and scarcity, and genes had to be “selfish” to retain market share in the human gene pool all that time. That is why even those who doubt or deny that all life emerged from evolutionary processes should recognize the merit in arguments from the selfish genes.

To govern our instincts well, we need to understand them. We cannot forget that evolution operates slowly, so our selfish genes are stuck in a time warp. Human instincts seem to be designed to help us survive and reproduce not under modern conditions but in “the environment of evolutionary adaptation,” that is, in the Stone Age. We like salt, sugar, and fat too much, because these elements of our diet were especially scarce in the Stone Age. Similarly, to generalize broadly, men enjoy video games and sports, because they were hunters in the Stone Age, while women, genetically adapted to be Stone Age gatherers, enjoy shopping.

Above all, the selfish genes care about sex, since that is how they propagate themselves. Evolutionary psychology is a rich source of testable, successful hypotheses about men and women and sexual desire and behavior.

The Sexual Double Standard

Men and women are different. They have to be, because men and women, in the Stone Age, faced very different reproductive opportunities, and their strategies had to adapt. Men’s costs to beget offspring are low (sex doesn’t take long), but females’ costs (pregnancy, childbirth, nursing, etc.) are high. Men need not economize their abundant semen, so they seek quantity of women.

Women, whose wombs are a scarce resource, seek quality. Women are naturally choosy, preferring mates with good genes and abundant resources. They are coy, reluctant to have sex except in the context of a committed relationship. Both strategies address the severe risk of abandonment by men, in whom “dad” instincts to guard and guide their offspring compete with “cad” instincts to score with as many woman as they can. Before mating, women instinctively seek commitment and parental investment from their mates. Women look to the future, focusing on affection and attachment.

Men’s main worry, if they do settle down to be dads, is to make sure the kids they are raising are really their own. That depends heavily on the sexual history and propensities of their mates. Men focus on physical fidelity and look to the present and the past. In particular, they instinctively prefer to marry virgins. Virginity signals that a woman doesn’t easily yield to seduction, isn’t yet pregnant with another man’s child, and has no sexual loyalties to rival those she will feel towards him. This instinctive male preference for virgins is the basis for the old sexual double standard, which regards a woman—but not a man—as dishonored by fornication.

The double standard offends modern sensibilities, and Christian sensibilities as well. Christianity has always risen above the sexual double standard by insisting that a man’s body belongs to his wife as much as hers belongs to him. Contraception, moreover, seems to rob the double standard of its rationale, since it allows women to have promiscuous sex without much risk of pregnancy. Similarly, DNA tests can ensure paternity certainty better than the old male strategies of virgin preference and sexual jealousy ever could.

But the selfish genes don’t know about modern sensibilities, or Christianity, or contraceptives, or DNA tests. They are very subtle after their fashion but absurdly out of date. They can’t inspire or tempt us to propagate them optimally under modern conditions. They feed us instincts suitable for the Stone Age, and reason and religion cannot erase the influence of instinct on most people’s behavior. And so the double standard remains.

In light of the double standard, the reasons why men and women should be chaste are different. Women should be chaste primarily from prudence. Men should be chaste primarily for the sake of justice.

Female Chastity as Prudence

For a woman, to have premarital sex is, first of all, to risk pregnancy as an unwed mother, with great costs and risks to herself and her child. Contraception can greatly mitigate this risk, though it doesn’t eliminate it; in a typical year, almost 5% of American women ages fifteen to forty-four unintentionally get pregnant.

Feminine instincts and bonding hormones can make a woman feel deeply—and perhaps unexpectedly—attached to her sexual partner and her unintended progeny. If her partner leaves her, she may suffer emotionally. If he is a poor mate for the long run, she may still find it hard to leave him. If she is surprised to discover that she wants to be a mother, he may not want to be a father. If she does leave him, he may get possessive, abusive, and violent.

Chastity is a good marriage market strategy. Cultural changes have probably weakened male virgin preference, but it would be rash for a woman to assume that it has completely disappeared and won’t come back. If she does marry, her future husband will probably be grateful if she is a virgin. If she is not, he may secretly wish that she had been and suffer from jealousy.

With all these downsides, premarital sex is imprudent for women. A secondary reason for women to be chaste is to avoid disappointing parents and siblings, who instinctively value her chastity.

Male Chastity as Justice

A man should avoid premarital sex principally to avoid injustice to women. Justice is giving what is owed. The feeling that a man owes something to a woman he has had sex with, even if there was no explicit quid pro quo, is a stubborn one, of which Jackson Browne’s song “Call it a Loan” is one of many poignant expressions. It stands to reason that the nature of that debt varies, depending on what was said and left unsaid between them, what society’s expectations are, and so forth. But in general, we know from evolutionary psychology that sex makes women feel a sense of attachment and belonging. A man who causes this feeling in a woman becomes responsible for fulfilling it.

Men are tempted to exploit women for pleasure and prestige, and need to be on their guard against this temptation. Exploitation is worst when the woman is underage or drunk or emotionally unstable, or when the man uses a position of power to intimidate her, tells lies to impress her, promises to marry her, conceals his marriage to someone else, gets her pregnant, or exposes her to a sexually transmitted disease. But the bottom line is that if he serves his own pleasure at the expense of her welfare, that’s exploitation. If he knew, or could have known if he thought about it, that she’d regret it the morning after, that’s exploitation. And if he knew, or could have known, that she’d regret it one year, or five years, or fifteen years later, when she’s wasted some or most or all of her remaining reproductive years on a guy who wouldn’t marry her, that’s exploitation, too. “He used me” is a standard—and just, and accurate—complaint made by women against men they’ve had sex with.

“Consent” is the standard defense of the modern seducer, but it’s inadequate. What did she consent to? He can’t assume it was just sex, full stop. Women are instinctively forward-looking. She probably wanted more, even if she didn’t articulate it. Her instincts will want more even if she did not consciously want it at the time. It’s not fair to expect her to have kept track of all his words and to ask no more of him than he explicitly promised. To do so is to insist that pillow talk be treated as seriously as the language of lawyers negotiating a contract. A just man must recognize that lovers’ talk is a kind of verbal foreplay, a sublimation of sexual desire, often beautiful and well worth enjoying, but not capable of dissolving duties founded on the more permanent and objective facts of sexual instinct. The only ethically safe course is either to marry a woman or else to leave her chastity intact.

The Magic of Marriage

Humans are intensely ambivalent about sex, regarding it by turns as vulgar, gross, and unseemly, or as sublime and beautiful. We place rape among the worst of crimes, while romantic love is one of life’s crowning glories, the theme of half the novels and songs the human race has written. The deceit and damage involved in so much premarital sex—cool dude bangs insecure girl and turns her into a single mom on welfare for life—fully justifies the repugnance that is one side of this ambivalence.

On the other side is the glory of marriage, and while there’s more to that glory than the selfish genes can explain, they shed an important light on it. For when two people marry, “leaving father and mother” as the Bible says and committing to lifelong monogamy, their genetic interests are united, at least approximately, creating a harmony of instincts. Ordinarily, our instincts put us in competition with our fellow human beings. In marriage, instinct is on the side of love.

Children are the large, obvious reason why marriage is good for society and why premarital sex isn’t. Sexual relationships always absorb a lot of people’s energy and attention, so they impoverish society unless they give something back. Marriage makes the next generation, under the most favorable conditions. Premarital sex is usually not intended for procreation, and if it does result in children, they enter life at a disadvantage because they lack stable parental commitments to raising them.

But even compared to childless marriage, premarital sex has an unwholesome character because, by failing to address genetic conflicts of interest through marriage, it allows competition, exploitation, and fear of betrayal to penetrate into the heart of the most intimate human relationships, not stealthily, but openly and as if by right. There is no way to make premarital sex promote the good of society or of the individuals involved. The world would be a better place if it never happened at all.