If you go back and read a bunch of biographies of people born 100 to 150 years ago, you notice a few things that were more common then than now.

First, many more families suffered the loss of a child, which had a devastating and historically underappreciated impact on their overall worldviews.

Second, and maybe related, many more children grew up in cold and emotionally distant homes, where fathers, in particular, barely knew their children and found it impossible to express their love for them.

It wasn’t only parents who were emotionally diffident; it was the people who studied them. In 1938, a group of researchers began an intensive study of 268 students at Harvard University. The plan was to track them through their entire lives, measuring, testing and interviewing them every few years to see how lives develop.

In the 1930s and 1940s, the researchers didn’t pay much attention to the men’s relationships. Instead, following the intellectual fashions of the day, they paid a lot of attention to the men’s physiognomy. Did they have a “masculine” body type? Did they show signs of vigorous genetic endowments?

But as this study — the Grant Study — progressed, the power of relationships became clear. The men who grew up in homes with warm parents were much more likely to become first lieutenants and majors in World War II. The men who grew up in cold, barren homes were much more likely to finish the war as privates.

Body type was useless as a predictor of how the men would fare in life. So was birth order or political affiliation. Even social class had a limited effect. But having a warm childhood was powerful. As George Vaillant, the study director, sums it up in “Triumphs of Experience,” his most recent summary of the research, “It was the capacity for intimate relationships that predicted flourishing in all aspects of these men’s lives.”

Of the 31 men in the study incapable of establishing intimate bonds, only four are still alive. Of those who were better at forming relationships, more than a third are living.

It’s not that the men who flourished had perfect childhoods. Rather, as Vaillant puts it, “What goes right is more important than what goes wrong.” The positive effect of one loving relative, mentor or friend can overwhelm the negative effects of the bad things that happen.

In case after case, the magic formula is capacity for intimacy combined with persistence, discipline, order and dependability. The men who could be affectionate about people and organized about things had very enjoyable lives.

But a childhood does not totally determine a life. The beauty of the Grant Study is that, as Vaillant emphasizes, it has followed its subjects for nine decades. The big finding is that you can teach an old dog new tricks. The men kept changing all the way through, even in their 80s and 90s.

One man in the study paid his way through Harvard by working as a psychiatric attendant. He slept from 6 p.m. to midnight. Worked the night shift at a hospital, then biked to class by 8 in the morning. After college, he tried his hand at theater. He did not succeed, and, at age 40, he saw himself as “mediocre and without imagination.” His middle years were professionally and maritally unhappy.

But, as he got older, he became less emotionally inhibited. In old age, he became a successful actor, playing roles like King Lear. He got married at 78. By 86, the only medicine he was taking was Viagra. He lived to 96.

Another subject grew up feeling that he “didn’t know either parent very well.” At 19, he wrote, “I don’t find it easy to make friends.” At 39, he wrote, “I feel lonely, rootless and disoriented.” At 50, he had basically given up trying to socialize and was trapped in an unhappy marriage.

But, as he aged, he changed. He became the president of his nursing home. He had girlfriends after the death of his first wife and then remarried. He didn’t turn into a social butterfly, but life was better.

The men of the Grant Study frequently became more emotionally attuned as they aged, more adept at recognizing and expressing emotion. Part of the explanation is biological. People, especially men, become more aware of their emotions as they get older.

Part of this is probably historical. Over the past half-century or so, American culture has become more attuned to the power of relationships. Masculinity has changed, at least a bit.

The so-called Flynn Effect describes the rise in measured I.Q. scores over the decades. Perhaps we could invent something called the Grant Effect, on the improvement of mass emotional intelligence over the decades. This gradual change might be one of the greatest contributors to progress and well-being that we’ve experienced in our lifetimes.