> It’s tempting to say that the lonely were born that way—it’d let the rest of us off the hook. And, as it turns out, we’d be about half right, because loneliness is about half heritable. A longitudinal study of more than 8,000 identical Dutch twins found that, if one twin reported feeling lonely and unloved, the other twin would report the same thing 48 percent of the time. This figure held so steady across the pairs of twins—young or old, male or female, notwithstanding different upbringings—that researchers concluded that it had to reflect genetic, not environmental, influence. To understand what it means for a personality trait to have 48 percent heritability, consider that the influence of genes on a purely physical trait is 100 percent. Children get the color of their eyes from their parents, and that is that. But although genes may predispose children toward loneliness, they do not account for everything that makes them grow up lonely. Fifty-two percent of that comes from the world.

Correct me if I'm wrong, but I think the math is wrong. Assume prior for being lonely is 50/50. Now, if 100% of twins reported that they are also lonely, this would be proof that loneliness is genetic. If it was 0%, it would imply some sadistic inverse relationship, but one that'd still be genetic. If it's 50%, the same as the prior, it means no relationship. 48% is pretty close.