That is pretty much the most basic split in psychology, with a few die-hards on either end (same as it ever was) and most in the middle arguing about the percentage split between the two.
One one hand we do like to view ourselves as independent creatures strolling though life making decisions based on our reason and our sense of morality.
So, its very possible that who we are and what we think is a result of the environment we are put into. And, thus, given a set of environmental factors outcomes should be pretty set.
And yet...
I know people - and I'm sure you do too - who came from all sorts of horrible situations and crappy childhoods, and messed up homes and all that - and for the most part they are OK people (no better or worse, no more messed up, then the average).
I also know a few people, from time to time myself included, who were more like the guy in the Merle Haggard song:
One and only rebel child,
From a family, meek and mild:
My Mama seemed to know what lay in store.
Despite all my Sunday learning,
Towards the bad, I kept on turning.
'Til Mama couldn't hold me anymore.
If its nurture, shouldn't kids from the same family be more like each other even more than they are? But wait, they are sharing a set of genes too - how come they are not identical?
On the other hand, we do know that stuff like the color of the eyes and hair, body type, and a lot of other stuff seems to come more from nature - from genetics - then from any choice we might make. Why not genetics for traits in behaviors, attitudes, thoughts even?
Most obvious difference in Nature of sex, is of course, sex. Both in ability and its role in procreation. Generaly we could say that on the whole, speices-wide femle humans are smaller, softer, different hair texture (and a huge, HUGE difference, one that we only barely pretend to understand - a fundamental difference in internal chemistry). These are all traits tied to genetics. As a result of those differences are there then, systematic differences in behaviors? Are there basic differences in abilities, skills, learning processes that are hardwired by genetics?
Beyond all that... Is all of this changing? Is that change being internally directed - genetic changes we brought about (intentional or unintentional) - or is that change a way the environment pushes on us and has us in its control? Could it be that society and 'civilization' itself somehow demands these changes and is propelling them? Or are we just one more set of living things that are only a part of a much larger living world that has its own changes going on that effect us, but that we are not aware of.
And yet...
there are huge differences between then and now that play into all of this too. The nature, complexity and interaction of additional environmental factors added since that time is also unknown. (I'm thinking here constant exposure to a variety of radio and electromagnetic waves from high tension power lines to cell phones, the saturation of things like TV and its ability to shape a lot of this, and the massive toxic chemical bath that we are constantly in that is so complex its true nature might not ever be known.)
For myself, I know a lot of what I am I just somehow was. It was like that as far back as I can see and remember. I don't know why. I know it's true.
And yet....
I also know that a huge part of my life was shaped by my association with groups of people in my pre-teen, teen years, and beyond who accepted me and pretty much left me alone (but included) to pursue life it my own preclude way - and a lot of 'who I am' is shaped and informed by those notions and those pursuits.