In the early 1920s, a list of exceptionally bright children was assembled for a study about growing up as a genius. These individuals became known to psychologists, affectionately, as the “termites”, after the Stanford researcher – Professor Frederick Terman – who began the study. The termites completed many surveys over their lifetimes, and the vast majority of the group have now died.

Their death certificates reveal that those whose parents divorced before their 21st birthday lived four years fewer than those whose parents stayed together until at least that point. Male termites typically lived to 76 as opposed to 80; female termites made it to about 82, instead of 86.

Although this group was a bit different to the rest of us – they each scored a minimum of 135 in high school IQ tests – brains and possible dweebish tendencies don’t appear to have exacerbated the impact of parental divorce. Overall, the termites handled life no better or worse than the US population as a whole. They committed suicide, developed alcoholism and themselves got divorced at the national rates.

So if parental divorce reduced longevity among the termites, it suggested worrying things about how divorce might affect all kids in the long run. The research subjects’ early deaths were pinned on higher rates of smoking, perhaps indicating a greater lifelong psychological stress following parental divorce.

But is this finding relevant today? After all, the termites experienced their parents splitting up during a period when divorce was much less common and much more stigmatized than it is now. Moreover, since that time, the reasons for divorce and the profile of couples most likely to divorce have changed in all manner of ways. So it seems reasonable to expect that whatever may have led these termites to feel stress more intensely than the other termites is no longer part and parcel of watching your parents split.

Researchers have a lot of disagreements about divorce trends. Most agree that divorce is less common today due to a rise in the age at which people first marry. But some experts believe that divorce rates in the US reached a peak among couples who married in the 1970s, and ever since the 1980s marriages have been more enduring. Other researchers, working with a different set of surveys, counter that the drop-off in divorce in recent decades has been overstated. In their view, divorce seems to have declined among younger couples, but it doubled in the two decades before 2008 among people over 35.

Divorce used to be the preserve of the upper classes. But as it has become legally less onerous, and its financial and social costs have been reduced, lower class couples have composed an ever greater share of divorcees.

Similarly, a highly educated wife used to be linked to greater odds of divorce, but that pattern has at least weakened in European countries taken together, and reversed in the most developed ones. In the US, too, women with a college degree are today far less likely to divorce than women who haven’t surpassed a high school degree. College educated women are also happier in their marriages and with their family life.

But what do these patterns have to do with how much divorce affects kids later in life? To answer that question requires comparing divorce trends to divorcees’ kids’ outcomes over time – outcomes that can reveal their levels of stress and how they have responded to stress as they have matured.

Various studies attempt this, but none over as long a period as an analysis of Swedish children. Astonishingly, Swedish records allow for comparison of people born more than a century apart, since face-to-face interviews using the same set of questions have been posed to Swedes born from 1892 onwards. Interviewees have been asked about their living arrangements growing up, about the extent of parental discord that they recall, and all about their mental health issues into adulthood, from insomnia to depression.

Many divorce trends over the 20th century suggest that children, on average, should have experienced noticeably less distress over time from their parents’ marriage ending. As divorce has become more common, it has become more socially acceptable. Female employment has risen and the welfare state has grown, meaning that single mothers are comparatively more able to provide for their offspring today than in the past. Custody arrangements have also changed, and children whose parents divorced in recent years are more likely than ever to maintain relationships with both parents.

But shockingly, divorcees’ kids in Sweden have seen no improvements in their relative educational attainment and psychological wellbeing. To this day, they are worse off by these measures than kids whose parents stay together. (And Sweden is, of course, a country with a far more generous welfare state and more liberal attitudes than most.)

The ongoing gap in educational performance appears to be due to families of a lower economic class becoming more prone to splitting up over the decades. Kids born into lower income families have always tended to do worse at school, so that trend isn’t due to divorce per se.

But the stubbornly lower psychological wellbeing of Swedish divorcees’ kids can’t be pinned entirely on income. Socioeconomics may explain part of it, but, instead, lots of family arguments appear to leave long term traces.

In short, the impacts of parental divorce are often subtle and long lasting. From the Stanford geniuses to Swedes born in the 1990s, the evidence suggests that kids whose parents have or are about to split up need more support than we realize.