It seems as though the nature versus nurture argument will go on forever, even though we now know a great deal about how the two of them interact in order to affect human behavior.
In the March/April issue of the Psychotherapy Networker, the cover story is titled "The Great Attachment Debate: How important is early experience? The "debate" is over the issue of whether or not the quality of the relationship between babies and toddlers and their primary attachment figure has a profound effect on mental health and relationships when the child grows up. In particular, "attachment" refers to how secure the child feels and behaves with its primary caretaker.
The two sides of the "debate" in the issue are represented by Jerome Kagan, Ph.D. on the one hand, and the tandem of Alan Sroufe, Ph.D. and Daniel Siegel, M.D. on the other. Kagan researches the effects of inborn temperament, personality, and neurobiology. Strouffe is a developmental child psychologist. Siegel is a UCLA psychiatrist who wrote a highly influential book called The Developing Mind.
With all these factors at work, there is also an effect from what scientists refer to as chaos, in which small changes in initial conditions can lead to big differences in complex phenomena like human behavior later on. This is known as the butterfly effect: the presence or absence of a butterfly flapping its wings could lead to creation or absence of a hurricane.
I actually disagree somewhat and think that infants are more resilient than they seem to think. For example, say the mother had an untreated post-partum depression during a baby's early life, but then got treated and became far more attuned to the child. Chances are, any ill effects of the child's experience during the first two years of life would then be reversed.
In this vein, both sides also agree that human psychology can change depending on later experience. If it could not, then they would both have to think that psychotherapy would be a complete waste of time. They agree that neither biology nor parenting experience is destiny. I should certainly hope so. If we could not adapt to changing environments, our species would have died out eons ago.
Kagan also says another important thing which sometimes gets lost in nature versus nurture debates. Genetic influences on behavior do not determine later personality variables so much as limit them. This is easier to see with physical traits. No matter how much Danny DeVito might have trained as a young man, he would never have been able to swim as fast as Michael Phelps did. Wrong constitution! This does not mean, however, that training would not have improved DeVito's lap times.
A hidden assumption in the whole debate that drives me bonkers is that the most salient patterns in the primary relationship between children and their parents somehow no longer influence a child past the age of two. Or if you are a psychoanalyst, past the age of five. Attunement, invalidation, the interactions between the temperaments of parents and children - in short, all of the most salient aspects of their relationship - often continue on and on in slightly altered forms, almost until somebody dies.
If you study the "persistence" of temperament and the security of attachment from childhood to adulthood, and draw conclusions based on just what happened in the first two years of life, you ignore this fact. If a two year old has an insecure attachment with an unattuned mother, but is taken away from that mother and raised by someone else, you would get a very different view of this "persistence." Similarly, if you limit the effects of attachment to only early attachment, you are ignoring the effects of extremely important family dynamics all through childhood and early adulthood.
Both sides in the "debate" define attachment the same way. Since controlling for all the variables is impossible, findings in the research literature will often conflict with each other. Depending on how studies were structured, it is easy to find studies that over-estimate the importance of attachment in the first two years, and others that under-estimate it. If on the other hand you assume that the early influences might continue on in time, and determine what they actually were, the "debate" all but disappears.