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Like many traits, shyness and boldness seem to be at least partly heritable; if an individual's father or mother was bold, chances are that it will be too, even if it is reared apart from its parents. Using fruit flies, Marla Sokolowski has been able to find not only a single gene, but its coded protein that lies behind a tendency to either move around as a larval fly (rovers) or have a more couch potato-like persona (called, reasonably enough, a sitter). Humans have also been able to domesticate breeds of horses, dogs, and other animals that possess not only particular body types but also characteristic behavioral traits, including willingness to fetch (retrievers) and aggressiveness (think pit bull), which means that such characteristics must be able to be passed from parents to offspring. Bold and shy people have different responses in their brains when they are presented with the same photographs of familiar or unfamiliar people, suggesting that these differences are an integral part of our makeup.

But as is also the case for many traits, the environment affects how much boldness or shyness (or any other aspect of personality) is expressed. Early experiences such as how much a mother interacts with her offspring can modify the tendency for an animal or person to be reckless or reserved, docile or rebellious. For many insects that lack any parental care, later behavior can still be affected by the place where a mother lays her eggs. In Wilson's sunfish, boldness means that you explore a new object in your pond sooner than the other fish do, and you are more willing to come out of hiding when a predator approaches. These differences between bold and shy individuals persisted in the wild for as long as Wilson and his crew were willing to look for them. And immediately after they were brought into aquaria in the lab, the bold fish were more willing to eat fish flakes, a novel food, instead of sulking in a corner, dreaming of scrumptious snails. But after a few weeks of getting used to the glass and plastic of their new digs, the distinction between the two types disappeared, and the formerly bold and shy sunfish were equally likely to approach a new object. The real world, it seems, keeps us—or at least some animals—different. These results make it tempting to speculate about the homogenizing effects of institutions such as prisons, or maybe even just urban living, on us humans; but of course we don't have a similar controlled experiment on people to use for comparison.

This universality of personalities across many different kinds of animals, including insects with their tiny brains, has two crucial implications. First, it means that the mechanism behind personalities can't be all that important, or at least that different parts of our physiology must account for the existence of personalities in different groups. In people and other mammals, we attribute, maybe even excuse, being laid-back or anxious to our hormones. Our stress levels are up because of cortisol or adrenaline, our neighbor is phlegmatic because his testosterone has decreased as he's aged. It isn't that hormones cause us to have particular traits, but that we have to have some physiological manifestation of our psychic differences. Even things that seem to be all in our head have to come from somewhere in the body, whether that is hormones coursing in the blood or electrical signals leaping in the brain.

Invertebrates, however, lack the same kind of hormonal system that mammals have, so their tendency to dart across a pond or cower under a leaf must arise from a different physiological source. The hormones are a handy means to an end, but they are not the only one. If fish, ants, and crickets have personalities too, we have to look somewhere other than our vertebrate types of tissues and organs for where they come from.

Second, when something seems to have evolved independently over and over again, such as wings in bats, birds, and butterflies, you have to suspect that nature is onto a good thing.