When a friend opts to date someone who is old enough to be their parent, a common response is to see if the decision passes the “half their age plus seven” test. The rule, whose origins remain mysterious, has been passed down through generations as a way of justifying or, more commonly, pouring scorn on other people’s couplings. It’s fine for a 22-year-old to go out with an 18-year-old. It’s not OK for a 38-year-old to go out with a 23-year-old, however a 26-year-old would be fair game. The older you get, the wider the permissible age gap: a 50-year-old can venture as old as 86.

That’s the theory. In practice, research conducted by Christian Rudder, co-founder of OKCupid, a dating website, suggests that when it comes to age gaps, men and women have slightly different ideas. While female users look for men roughly the same age as them (or perhaps a year or two older) men prefer women in their early twenties, regardless of their own age. While women prefer a small and constant age gap, men are so hooked on the idea of a nubile young partner that they prefer a larger age gap the older they get.

Are men searching wisely? In theory there are plenty of reasons to favour a small age gap. The ability of both members of a couple to sing a favourite childhood television theme tune could bond them together, at the risk of irritating those nearby. And sharing the joy of getting a free bus pass at a similar time, or saving money from combining big birthday parties, is clearly attractive.

Some economists have wondered whether smaller age gaps between partners could have wider, societal benefits, as they might help to narrow the gender earnings gap. Because earnings rise with age, and women tend to couple with older men, relative earnings around the time of childbirth could put subtle pressure on women to drop out of work. That said, a study that compared Danish twin sisters found that the earnings of women who married older men were no different, on average, than those who married men closer to them in age.

Could a smaller age gap also make couples more likely to stay together? In 2014, the Atlantic claimed that “a five year age difference makes a couple 18 percent more likely to get divorced, compared to a couple born on or around the same year.” While the study cited – which polled American couples and ex-couples – did show an association between divorce rates and age gaps, it did not prove a causal link. Something about the kind of person who opts into a marriage with a large age gap could be driving the higher divorce rates, rather than the age gap itself. A bright young thing considering a silver fox should also take heart from a study by Britain’s Office of National Statistics. It did not find a strong link between age gaps and divorce rates in England and Wales, though there was some evidence that women marrying later than 30 who were more than ten years older than their spouse were more likely to divorce.

Common sense does suggest that a large age gap would have implications for old age. Having someone to look after you in your dotage is wise, as is avoiding widowhood. A younger, healthier partner could make sense, at least from your side of the equation. Another study by Sven Drefahl of the University of Stockholm looked at people over the age of 50 in Denmark, and found that men with younger spouses survived for longer than those with ones of a similar age. The older their spouse, the worse their survival chances, even after controlling for things like education and wealth. Again, the link might not be causal: healthy men might be particularly able both to attract younger mates and live to a ripe old age. But mysteriously, this phenomenon does not appear to apply to women, where the bigger the age gap, the worse their survival chances, regardless of whether they were younger or older. In the case of women with younger husbands, Drefahl suggested, the gender difference could be due to women being less reliant on their partner for support, and so benefiting less from the energies of a younger spouse.

What evidence there is, therefore, vindicates the choices of OKCupid’s users: women should pick men who are as close as possible in age to them, while men should look for younger women. A true economist, however, would look for better evidence, perhaps by comparing the marital bliss of random couples with varying age differences. Unfortunately for them, but luckily for the rest of us, people make their own choices – and are free to ignore silly rules of thumb.