A reader of this blog recently asked me to comment on the ideas discussed in the book "The nurture assumption", by Judith Rich Harris. Harris's basic message is that the power of parenting to shape personality is mythical. One of the things that "everybody knows", but which may not be true, is that your parents make you the person you are; Harris discusses information that suggests that this is not as true as "everybody" thinks. She debunks the claim that the nurturing you experience is your most powerful shaper, and suggests instead that what children experience with their peers is the most essential factor in personality development. This conclusion has been praised as a brilliant insight into the way our prima facie assumptions can blind us, but on the other hand has been panned as cherry-picking data to support a premature conclusion.
The "true facts" about this matter are going to be elusive, but let me make a few relevant comments. First, I'd suggest that one of the reasons that "everybody" believes in nurture effects is that they also believe in an overarching assumption about the importance of timing of experiences. This latter assumption, called "infant determinism", is the belief (difficult to test or support) that experiences that occur in the first months and years of life have a more powerful influence on the individual than later events. This "seductive idea" (addressed by Jerome Kagan in his book, Three seductive ideas) is almost universally accepted in Western thought, with the notable exception that young children are sometimes said to be too young to remember unpleasant events. The connection with the nurture assumption, of course, is that the infant years are the period when nurturing places most demands on parents, and when poor care practices have the most immediate and obvious impacts on the child.
A second issue in parsing the nurture assumption is that there is an obvious alternative to the effect of nurture: that children's genetic make-up determines their personalities and abilities. But these two factors, heredity and experience, do not always exist independently of each other, and it can be quite hard to tease them apart. For example, one type of relationship between genetic characteristics and experience is evocative in nature. This means that individuals with certain genetic characteristics may behave in ways that cause them to have atypical experiences. An unusually active, rough-and-tumble child may have more physical injuries and more disapproval from adults than a quieter individual. Children do not simply receive nurture passively, but actively shape the experiences they have with parents and with the rest of the world.
Curiously, even people who believe strongly in the nurture assumption will often blame bad behavior on "bad company", "getting in with the wrong crowd", and "peer pressure". But the evidence is that children and teenagers who associate with "bad elements" have usually been rejected by more normally-behaved children and have found friends among other rejected children. Certainly members of each of these groups will learn from each other once they begin to associate, but the important question is, what made them associate to begin with?
An important point about Harris's conclusion (peers more influential, parents less) is that there are some behaviors and skills that children are likely to learn mostly from peers. For example, a parent may tolerantly participate in negotiation about a child's bed time, but in fact the parent holds so much of the power that the negotiation is only superficial. Child-to-child negotiation is the real thing, with two people of similar resources doing their best to figure out a compromise that will give them part of what they really want. Parents may let a child cheat at a game, or tell a transparent lie, but other children will be exceedingly critical and may even refuse to associate with someone who repeatedly does these things. Children are the ethical experts who tell each other when secrets may or may not be told; later, they learn adult standards, but from other children they learn that standards exist and may even preclude telling your mother how you got hurt.
In my opinion, "The nurture assumption" set up a man made of the same straw as that formerly used in the "nature versus nurture" argument. All sorts of things are important during development, but this is not to say that all of them are equally important at every developmental stage. We can't reject a factor altogether because it does not seem influential at a particular period of development. The real question may be: what kind of experiences, occurring when, and in combination with what genetic characteristics, create adult personality? This we can answer only gradually, as we accumulate relevant evidence.
[Incidentally, as I looked at Harris's bio, I saw that her early work was on right-left reversing prisms, a topic related to the much-mentioned rod-and-frame test on my CV. Small world!]