quiet-the-power-of-introverts-in-a-world-that-cant-stop-talking-susan-cain

share only 50 percent of their genes on
average. So if you measure introversion
or extroversion levels in pairs of twins
and find more correlation in identical
twins than in fraternal pairs—which
scientists do, in study after study, even
of twins raised in separate households—you
can reasonably conclude
that the trait has some genetic basis.
None of these studies is perfect, but
the results have consistently suggested
that introversion and extroversion, like
other major personality traits such as
agreeableness and conscientiousness,
are about 40 to 50 percent heritable.
But are biological explanations for introversion
wholly satisfying? When I
first read Kagan’s book Galen’s
Prophecy, I was so excited that I
couldn’t sleep. Here, inside these pages,
were my friends, my family, myself—all
of humanity, in fact!—neatly sorted
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