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Nature Vs. Nurture?

When my son, Alec, was born, I stood blissfully in the nurturist camp: I believed (as did every parent I knew) that environmental forces  -- parenting above all else  -- exerted a greater force on children than nature, or inborn traits, did. I assumed that heredity ruled our basic looks and our susceptibility to certain diseases. But our habits, foibles, values, and virtues? I thought that those were almost purely nurture, shaped by love and discipline.

Over the next couple of years, however, a few bombshells hit the zeitgeist. Just after my son turned 1, a conference on infant brain development led to a spate of news reports stating that the very wiring of our babies' minds is in our hands: The more time parents spend singing and talking to them in the first year, the more complex babies' neural pathways become, presumably leading to more success in life. Love is important, but an urgently timed barrage of words and eye contact may have greater impact.

Then, when Alec was 2, came The Nurture Assumption, a book declaring that children turn out the way they do mostly because of genes and peers. According to author Judith Rich Harris, parents matter less than they think.

In a one-two punch of hyperpublicity, these events left many parents with whiplash, feeling anxious (did we miss our window of opportunity?) and helpless (so they're learning right from wrong in the sandbox? That is, if they weren't born with psychopathic DNA).

We know that every child's temperament is significantly swayed by genes. Behavioral geneticists have been studying this link for decades. But does this fact bring our power as parents under attack, or is this an opportunity to rethink our role without seeing it as diminished?

Genetics 101

Back in high school biology, most of us were too busy giggling over the larger picture of sperm meets egg to absorb the scientific details of conception. To refresh your memory, this is the moment when, if the dance goes smoothly, 23 of Dad's chromosomes waltz off with 23 of Mom's. Divided unevenly among these 46 chromosomes are roughly 100,000 genes.

Nearly all genes travel in pairs, and some of our traits  -- whether you'll have a cleft or a rounded chin  -- emerge when one gene is dominant over its partner. For recessive traits, such as blood type O, to appear, you must inherit the same recessive gene from both parents. Other characteristics, like blood type AB, result when certain genes are codominant. But many of our physical traits, like eye color, are influenced by many genes, not just one pair.

Blood type and eye color are examples of traits that are 100 percent heritable. Height, by contrast, is between 80 and 90 percent heritable, which essentially means that environment  -- everything from conditions in the womb to childhood nutrition  -- accounts for 10 to 20 percent of the differences in height. Also, genes sometimes have what's called an additive effect: Rather than expressing, say, "tall" genes from Mom's family or "short" ones from Dad's, most children grow to a kind of genetic average.

The Roots of Personality

Studies suggest that many temperamental and behavioral tendencies are ultimately 30 to 50 percent genetic. Five major personality traits  -- called the Big 5  -- show the strongest influence: extroversion, agreeableness, neuroticism, conscientiousness, and openness to experience. Others with a significant genetic legacy include altruism, shyness, accident-proneness, and even self-esteem.

Are such tendencies present even in utero? Kimberly Saudino, Ph.D., an associate professor of psychology at Boston University who studies infant and toddler twins, says that in the first few years, there's genetic influence on three broad personality traits: activity (energy, vigor, and movement), task orientation (attention and persistence), and affect-extroversion (emotionality and sociability). Interestingly, where similar adult traits show close to 50 percent genetic influence, in babies these characteristics are only 20 to 40 percent heritable, meaning that in babyhood personality tends to be more influenced by environment.

Why? Not all genes express themselves from birth. Think of those that trigger sexual development  -- they don't switch on until your teens. When it comes to personality, Nancy Segal, Ph.D., author of Entwined Lives: Twins and What They Tell Us About Human Behavior, and her colleagues have observed that the influence of genes increases as we age.

Julia Glass is a freelance writer. Her novel Three Junes won the 2002 National Book Award for fiction.