What makes us who we are?
It wasn't until the 1970s that behavioural geneticists worked out productive techniques for answering questions about nature vs nurture. One method involved looking at adopted children, whose genes were provided by one set of parents and whose environment was provided by a different set. Another method involved finding identical twins separated at birth: same genes, different environments. A third involved comparisons between identical twins and fraternal twins reared in the same family (identical twins have the same genes; fraternal twins are genetically as different as ordinary siblings). Other research designs made use of the genetic differences between ordinary siblings, half-siblings, and step or adoptive siblings raised in the same family.
None of these methods is perfect, but they each have different flaws. It is therefore noteworthy that they all produced essentially the same results. Two results, actually—one surprising, the other not.
The unsurprising result was that genes matter. Since the 1970s, behavioural geneticists have measured many different human characteristics in many ways. They've looked at personality traits such as extroversion, conscientiousness and aggressiveness. They've looked at mental disorders, intelligence and aspects of people's life histories (such as careers). In virtually every case, the results were the same. About half the variation in the measured characteristic—the differences from one person to another—could be attributed to differences in their genes.
The surprising result had to do with the environment. Since genetic effects account for only about half of the differences among us, the other half has to be the result of environmental effects, right? Well, that was the assumption. But researchers still haven't been able to pin down which aspects of the environment are important. All they've been able to determine is which aspects of the environment are not important. The aspects of the environment that don't seem to matter are all those that are shared by all the children who grow up in a given family—which includes most of the things the word "home" makes you think of. Whether the home is headed by one parent or two, whether the parents are happily married or constantly rowing, whether they believe in pushing their children to succeed or leaving them to find their own way in life, whether the home is filled with books or sports equipment, whether it is orderly or messy, a city flat or a farmhouse—the research shows, counterintuitively, that none of these things makes much difference. The child who grows up in the orderly, well-run home is, on average, no more conscientious as an adult than the one who grows up in the messy one. Or rather, he or she will be more conscientious only to the extent that this characteristic is inherited.
And the conclusion:
It is the relationship system, with its bulging storehouse of memories of Mum and Dad, that makes us believe that our parents played a central role in making us who we are. This system contributes more than its fair share to our conscious memories. The strong emotions associated with these memories make us feel that they must be important. But the truth is that people don't know why they turned out the way they did. Many of the important things that happen during development go on outside the purview of the conscious mind. Asking people why they are the way they are may produce interesting answers, but we should not place much weight on them.
Asking their parents is even less likely to be profitable, because parents see only one part of their children's lives. Though relationships with parents greatly affect the day-to-day happiness of children, just as marital relationships greatly affect the day-to-day happiness of adults, neither leaves deep marks on the personality. In the long run, it is what happens to them outside the parental home that makes children turn out the way they do. After all, outside the parental home is where they are destined to spend their adult lives.
One of my favorite books is The Agile Gene: How Nature Turns on Nurture by Matt Ridley. His main thesis is that nature works via nurture (Nature via Nurture was the British title), and that the two are hard to distinguish. He puts a lot of evidence into it (much of it briefly discussed in the article, he's a big fan of the twin studies), and the entire book is quite convincing. The only thing that's a little problematic is that he likes to assign percentages to things, and some of them come off as being weird (like parental influence accounting for ~10% of intelligence, how can you estimate that?). This doesn't take away from his ultimately fantastic book.
I'm currently taking a class with Paul Ehrlich, (author of Human Natures: Genes, Cultures, and the Human Prospect), who likes to say that trying to differentiate between genetic and environmental contributions to personality are like trying to differentiate between the contributions of length and width to the area of a triangle; they're so intimately connected that it's impossible to separate them. He doesn't buy genetic causes for behaviors at any but the most basic level, and loves the "gene shortage" argument, that there simply aren't enough genes to program those behaviors (a position I vehemently disagree with in certain realms).
I fall a bit between those two. There are almost certainly (I'm certain about nothing) genetic causes for behaviors, but they almost certainly interact strongly with the environment. I think that Harris gets it damn close to right in her article (although I don't know how convincing the brain modules she talks about are).
Anyway, this is a fantastically complicated and fundamentally interesting topic. What makes us who we are? We're closing to knowing than we ever were before, and we're almost certainly nowhere near understanding it all.