The average husband is older than the average wife. This is true in almost every country of the world, and has been for as long as we can find records. The distance isn’t huge: in the Western world, the most common age gap between married couples is only 2-3 years. But it exists.

Why? Is it healthy? And what are we to make of relationships with really substantial age gaps?

The "nature" explanation

In the majority of human relationships, only women can carry children. That means men are typically tasked with providing income for the household – and the older a man is, the more he earns. That means that in theory, it’s sensible for women to seek out older men as prospective fathers. This is what we’ll refer to as the evolutionary psychology perspective.

Of course, we’re not always hyper-rational when it comes to love. Fortunately, though, our instincts are (partially) on our side: one of the biggest studies on the matter – spanning 37 cultures and 10,047 participants – found that women are consistently attracted to older men, and men to younger women. A second, even larger study found the same thing, and those preferences are reflected in marriage records too.

That said, the male/female age discrepancy is changing over time, as couples featuring an older woman become more common. This is roughly what we’d expect in developed countries: large age differences between spouses have been associated with more patriarchal family systems and less romantic love.‍

So far, so sensible. But no article on age-gap relationships would be complete without a section on the following charts, published somewhat infamously by Christian Rudder of OKCupid.

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The male chart is predictable: no matter how old a man is, he prefers a female partner to be around 21 years old. This is just before the beginning of a woman’s most fertile years, which last roughly from one’s mid twenties to one’s mid thirties. As we’ll see later, women outside these age groups experience higher volumes of chromosomal errors, which contribute to higher rates of miscarriage.

The female chart, however, shows that women are primarily interested in older men until the age of about 30 – at which point they prefer their partners to be either the same age or slightly younger than them.

The evolutionary psychology perspective outlined above explains this. Men seek younger and more fertile mates to ensure reproductive success; women seek high-status males who are able to provide resources over the long term in order to ensure survival of offspring.

Nonetheless, the average age of marriage in the UK and US hovers around 30, though has historically been younger. 30 is an age at which women prefer roughly same-age partners and men prefer younger partners. The compromise is that women marry men who are slightly older than them, and men marry women who are (only) slightly younger.

The "nurture" explanation

But there’s something that doesn’t make sense. If this effect were just an outgrowth of evolutionary processes, we’d expect to see something similar in other human-like mammals. But when we look at chimpanzees, our closest living relatives, we see the opposite result. Male chimpanzees consistently prefer their mates to be older. In fact the older a female chimpanzee, the more social cachet her mating partner is likely to possess.

Perhaps this is to be expected: young female chimpanzees often have no track record of successful childbirth, and indeed first-time chimpanzee mothers have higher rates of neonatal mortality (that is, their infants are more likely to die).

But the same is true in human females: women under 25 have a significantly higher risk of miscarriage than those aged 25-35, and infants are more likely to be stillborn to mothers aged 24 or younger than they are to mothers aged 25-39.

So why don’t we see similar preferences in human males? One factor could be menopause: we don’t know very much about menopause in chimpanzees, and particularly not in the wild. Scientists’ best guess is that chimpanzees tend to undergo menopause at about 35–40, a bit earlier than human females – although women tend to live for several decades longer than female chimpanzees. Nonetheless, in chimpanzees just as in humans, increased maternal age is associated with decreased fertility (when fertility is understood as “ability to deliver a surviving infant,” rather than just “ability to conceive a pregnancy”).

So very young and very old chimpanzees are less fertile, just as human women are. Why, then, do male chimpanzees prefer older females whilst male humans seek out younger women? There isn’t a clear biological explanation, so instead we must look for a cultural one.

Going back to the charts above, it’s worth noting that whilst men are most physically attracted to very young women, the age gap reflected in actual partner choice is much smaller. This may be due to the fact that young women are typically disinterested in much older men, and prefer someone their own age or a couple of years older – so even if older men tried to pair up with them, the average woman in her early 20s would refuse them. Exceptions can be seen when women are particularly vulnerable, or when men are particularly resource-rich (consider the case of Leonardo di Caprio).

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But it’s also likely due to preferences that go beyond the physical. Very few people seek partners based on appearance alone, and those who do are typically not used to long-term relationships. We usually want someone similar to us, with whom we can share jokes and experiences. Those criteria often aren’t satisfied by much younger or much older partners.

We usually want someone similar to us, with whom we can share jokes and experiences. Those criteria often aren’t satisfied by much younger or much older partners.

There’s also the fact that men seeking much younger partners are generally viewed as predatory, because they’re selecting for women with substantially less life experience (and, if under 25, reduced ability to plan for the long-term, make strategic decisions, or control their impulses). The same is true of women, although the ‘cougar’ phenomenon is poorly evidenced outside television and media. Irrespective of gender, people tend to believe that the older partner in the relationship is likely to receive more benefit from it than the younger person.

Another reason for our aversion to big age gaps might be that unlike in many other mammals, human parents are expected to stay in their children’s lives for as long as decades. Men live less long than women at the best of times, so advanced paternal age signals comparatively low suitability as a prospective parent. This might be part of the reason that big age gaps are more common in LGBTQ partnerships, in which biological parenthood involving both parties is less likely.

It’s also easy to forget that just as women’s fertility decreases after about 35, male sperm quality declines significantly with age. This, too, increases the likelihood of birth defects and chromosomal abnormalities. Men over 40 are about 30% less likely to be able to create a pregnancy than men under 30, even controlling for the age of the prospective mother.  And men over 45 take about 5 times as long to get their partners pregnant as men under 25 (also controlling for maternal age).

Men over 40 are also three times as likely to experience severe erectile dysfunction, meaning conception must be medically assisted.

So there are plenty of logical reasons we frown upon large age gaps in romantic relationships. But love isn’t always a rational creature, and sometimes we can’t help falling for someone much older or younger than ourselves. So what happens then?

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Your odds of success

There’s some evidence that larger age gaps than average are associated with higher divorce rates – and the bigger the age gap, the higher the likelihood of divorce. But as has been discussed elsewhere, people who end up with big age gaps in their relationships might just be more likely to divorce for another reason – perhaps there’s something in their personality or cultural background that causes both phenomena.

One thing we do know is that couples with age gaps of 4 years or more experience lower relationship satisfaction as their marriages mature. The bigger the age gap, the lower the satisfaction – despite the fact that both men and women see temporary happiness boosts from having a younger spouse. Married women in particular are happiest with partners of their own age, rather than with older men. This coincides with what we know about death rates: for men, having a younger spouse is linked to improved longevity, whereas for women, having a partner who is either considerably older or considerably younger tends to shorten lifespan, and the greater the age gap (in either direction) the shorter her life.

But something else we know is those who have been in age-gap relationships previously are likely to be more open to them in the future. This shouldn’t surprise us too much: humans often change their attitudes in order to justify past behaviour. But if age-gap relationships were often a powerfully negative experience, we’d still expect those involved to have negative reactions to them. And almost all of us can think of happy, successful relationships that have an age gap of 10 years or more.

In all, the evidence against relationships with large age gaps is moderate to substantial, but the evidence against relationships with small age gaps is modest to non-existent. And, of course, not everyone is average: perhaps you’re part of an outlier couple who can make a big age gap work in the long-term. But if you’re still hunting for your life partner, evidence suggests you’ll probably find them within 1-4 years of your own age.