2. Well, suppose we change the question to "how much of development is determined by nature?", admitting that it's not going to be all nature or all nurture?

That sounds easier, but in fact it's not that simple. Different aspects of development are influenced differently by nature and nurture, and there would have to be different answers for each of them. For instance, eye color is almost entirely a matter of genetics (although I recently read of a drug that changes eye pigmentation). The language you speak is a matter of environment, although of course the fact that you have the capacity to speak at all has strong genetic factors determining it.

Sometimes the effects of nature and of nurture are different when apparently similar aspects of development are at issue. For example, in mammals, the shape of the upper jaw is largely genetically determined, but the shape of the lower jaw is strongly influenced by experience with the environment. It's really important that the two jaws mesh properly for biting and chewing, and of course orthodontists were not part of the environment of early adaptation. The jaws had to develop to fit each other during periods in early individual development when the bones are growing quickly and are easily shaped by pressure. One jaw is shaped by pressure, the other isn't, or at least not much; the result is that the adult has a mouth in which the lower jaw fits well with the upper.

How much nature, how much nurture-- it all depends on what aspect of development you're talking about.

3. How can nature and nurture act together? Do they just take turns, or does nature do the first part of the job, and nurture go to work later?

Many aspects of development operate as what are called "transactional processes". These are different of the usual ways people think of things working-- for example, that a mother does things that influence a baby. In transactional processes, two entities influence each other more or less simultaneously, and each is changed by what happens; as they continue to affect each other, the kinds of effects each has will change. Together, they shape development.

Nature and nurture can act transactionally, too. First, the kind of nurture someone gets may depend on what nature has provided. For instance, a thin, quiet child may be allowed to sit on his mother's lap well into middle childhood, while a robust and active brother or sister may be pushed off early because the bouncing is so hard on mother's knees. A child with a difficult personality may self-select his or her diet down to less than ideal nutrition, whereas an "easier" child may eat what's put before him and be well-nourished.

But what is going to be "second" here? How can nurture change nature? It can do so, because events both inside and outside the body can cause genes to "turn on" and "turn off". An individual's genetic make-up is not the same at all times during life, so developmental options are also different.

4. It's not an easy thing to test a person's genes, but it ought to be easy to observe what someone experiences. We can do that, can't we? Or can we?

There are several tricky parts to studying nurture. One is that we don't necessarily know what experiences affect development at all, and it's even more difficult to know whether all experiences have different effects at different times of life. But in addition, there are experiences that at a casual glance seem negligible, but turn out to have a big impact.

For example, you've probably seen two things about young babies. One is that in the first months their hands are usually folded into fists, which the babies can't open all the way or use finger by finger; only much later does the baby open the hand or put thumb to forefinger in a pincer grip. Another is that from the age of a few months, a baby placed on its tummy puts out its hands to try to support and push its head and body up. It's not so obvious that the pushing up is the experience that helped the baby become able to open its hands and use fingers independently. In fact, the pressure the baby exerts against the palm while pushing up actually stretches the hand open and molds it into a more mature and usable form. This apparently trivial experience, based on the baby's own actions, makes the difference between a hand which doesn't open properly and one that works well for grasping and letting go.

Conclusion: The "nature-nurture question" is deceptively simple, and manages to avoid some of the real issues about how development is shaped. When we look at those issues, we see that different types of development can be vastly different in the rules that guide them. That's why I'm just on the side of studying how development works, not of nature or of nurture.