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

The interesting thing is that these differences
are heritable, not learned, so
that the offspring of bold guppies who
move into bad neighborhoods inherit
their parents’ boldness—even though
this puts them at a severe disadvantage
compared to their vigilant peers. It
doesn’t take long for their genes to
mutate, though, and descendants who
manage to survive tend to be careful
types. The same thing happens to vigilant
guppies when the pike suddenly
disappear; it takes about twenty years
for their descendants to evolve into fish
who act as if they haven’t a care in the
world.
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The trade-off theory seems to apply
equally to humans. Scientists have