Many films, novels or operas end with the marriage of a couple in love. If this is a modern work, the implied “happily ever after” includes a lot of sex. Yet for years, data has shown a gradual decline in sexual frequency during marriage. Folklore offered the “bottle theory” of marriage. According to this theory, if the couple puts a penny in a bottle each time they have sex during the first year of marriage, and then takes a penny out each time they have sex after their first anniversary , the bottle will never empty. The data suggests that the decline is not that severe, but it is unmistakable. Yet why?

We followed newlywed couples through the early years of their marriage. About every six months, they answered questionnaires about their sexual desires, their marital satisfaction and other factors. No two couples are the same, of course, but the general pattern is clear from both studies. During the first four or five years of marriage, the wife’s sexual desire declined steadily, while the husband’s showed no change. The same pattern was found for questions specifically about sexual desire with spouse and sexual desire with anyone in general. Five years after marriage, the husband’s average sex drive is the same as when he was walking down the aisle, but his wife’s drive has decreased.

Additionally, we found that marital satisfaction for both husband and wife deteriorated as the wife lost her sexual desire. (The husband’s sexual desire was irrelevant to anyone’s marital happiness.) Could wives lose sexual desire because the marriage goes wrong? No: Time lag analyzes indicated that her loss of desire occurred first, resulting in lower satisfaction later. Early levels of (dis)satisfaction did not predict how quickly wives lost interest in sex.

Above all, it was not due to childbirth. Becoming parents worsened the mismatch, as in a more pronounced decline in wives’ sexual desire. (In one study, but not the other, becoming a father significantly increased a man’s sexual desire!) Nor was it due to stress or depression, although these may have contributed in part. to the problem in some cases.

The folklore bottle theory aside, the first signs of this pattern have been noticed. Bettina Arndt, an Australian journalist, came up with the idea for a racy bestseller, which was to have couples keep a diary of their sex life. His book, sex diaries, reported some lively and creative practices, but the most common theme was that of husbands begging for sex while wives refuse. Pillow talk filled with memorable phrases like “get that thing away from me!” Arndt concluded, sadly, that something seems to drive women to “stop sex” once they’ve settled into a committed relationship.

One possible explanation that fits our data is that female sexual desire increases during the brief phase of passionate lovemaking. Nature may have arranged this as a way to encourage man to make a long-term commitment. Humans evolved from other great apes, but none of the other apes take fatherhood seriously, let alone provide daily food and shelter to the mother of their children for many years. Human men are remarkably different in this regard, and evolution had to engineer some serious changes to achieve this. Men’s sexual desire may have taken on an addictive character, so that men become attached and continue to hope for sex even when the yield of pleasure wanes, as in some drug addictions. Meanwhile, nature has arranged for women’s desire to temporarily increase during courtship. The couples in which the woman’s desire increased enough to make the man addicted produced more children than the others. We descend from them.

From this point of view, then, it is not that marriage is bad for women’s sexual desire. On the contrary, the phase of passionate love stimulates it, and when this type of love fades, sexual desire returns to the baseline.

The seeds of later dissatisfaction are thus sown by nature’s method of keeping couples together long enough to care for the children that sex will bring. The young man thinks he has found his sexual soul mate. Any reservations about the woman or her need to support herself seem minor compared to the prospect of enjoying that great sex every day for the rest of her life. When she stops wanting to have sex, he may think she has changed unfairly. After all, her sexual desire is the same as it always has been.

During this time, the wife may be as surprised as her husband by her loss of desire. For a year or two of intense passion, she wanted sex almost as much as he did. Above all, she never agreed to have sex when she didn’t feel like it. When his desire wanes, she may think he’s being unfair because he wants her to offer him sex even when she’s not in the mood. He didn’t ask for that during the passionate courtship. (Granted, he didn’t have to.) It can be difficult for her to deal with the unexpected issues of her lack of desire, her continued sexual desire, and the feeling that the quality of her marriage is suddenly dropping. She may or may not make the connection between her loss of sexual passion and the mutual feeling that neither is as satisfied with the marriage as before. After all, it only emerged from our large data set.

The decline in happiness in marriage is a serious problem. I suspect it’s because when the woman starts losing sex drive, both parties think it indicates a problem with their marriage, and they blame or blame each other. Psychology could strengthen marriages if it could help people realize that this is a common pattern. Rather than being a sign that the marriage is in trouble, they might see it as a standard problem that they can fix as a couple.

A future article will discuss how couples might deal with this.