How and why do we develop such distinct personalities?

Judith Kleinfeld

When I took my three children to a Chinese shop in San Francisco, the owner asked in astonishment, "Are these children from the same family?"

I could understand his surprise. My oldest boy was aloof, dressed in black with an Afro hairstyle. My daughter was prim and proper in her blue sailor dress. My youngest son was unusually outgoing, immediately engaging the store owner in an animated conversation.

Many parents are surprised at how different their children are. They have lived in the same homes, were raised by the same parents, but their personalities are so different.

What is personality anyway and how do such differences develop?

In his new book ("Making Sense of People, Decoding the Mysteries of Personality"), neuroscientist and psychologist Samuel Barondes answers these questions.

Studies across cultures have identified similar traits which make up personality. They are called "The Big Five." Check the Internet and you'll find many sites which let you rank yourself on these traits:

• Extroversion: These people are warm and friendly, cheerful and prone to be happy.

• Agreeableness: These are trusting people who prefer compromise to argument.

• Conscientiousness: These are people who are well-organized, reliable, and accomplish things.

• Neuroticism: These are people who are anxious, who collapse under pressure, and are prone to depression.

• Openness: These are people who like to try new thing, play with ideas, and buck convention.

It is tempting to think some of these traits are good, like agreeableness, and others, like neuroticism, are bad. But this is not the case.

Agreeable people are cooperative and make alliances. But people high on this trait are often taken advantage of.

"Studies show that people who rank higher in agreeableness tend to earn less money, even though they are valued as team players," writes Barondes. "In contrast, those who are lowest in agreeableness are more likely to rise to the top of their fields."

Neuroticism brings painful emotions, but neurotic people are often high achievers and creative when their emotions do not overwhelm them with distress.

These differences in personality are mostly genetic. In "Songs of Innocence," the poet William Blake gets the science close to right.

Every night and every morn

Some to misery are born.

Every morn and every night

Some are born to sweet delight.

The importance of genetics reveals itself in studies of identical twins raised together and raised apart. In a famous study of 100 identical twins, psychologist Thomas Bourchard and his colleagues found that identical twins separated at birth and raised in different kinds of families turned out to be very similar in personality.

Scientists put the influence of genetics at about 50 percent. But environment can also have strong effects because environment can actually alter the expression of our genes.

One of the most interesting examples is Michael Meaney's studies of mothering on the personalities of rat pups. Rats lick their babies. Some rat pups were raised by high-licking rats (like affectionate loving parents) while others were raised by low-licking rats (like detached parents).

The highly licked pups turned out to be less fearful than those raised by low lickers. They explored their cages and had lower levels of stress-related hormones.

Then Meaney swapped the pups born to high-licking mothers to low-licking mothers and swapped the pups born to low-licking mothers to high-licking mothers.

The quality of licking changed the personalities of these pups. The high-licking mothers turned the genetically fearful pups into stress-resistant pups. Licking had affected the activity of genes. These changes persisted throughout the pups' lives, shifting the setting of the brain that controlled the pups' responses to stress.

Experiences, especially early in life, can have sustained effects on personality, through changing the activity of genes.

Biology is not destiny. Experience works through genes, changing DNA, which in turn changes personality.

My children seem to have converged as they have conformed to the environments of adulthood. But they're still very different people.

Judith Kleinfeld holds a doctorate from Harvard and is professor emeritus of psychology at the University of Alaska Fairbanks.

Judith Kleinfeld