Since having a child, and seeing my friends and sibs having children I have come to the conclusion that there is a large degree of our personalities are "hard-wired". It's amazing how each child comes out differently with their own personality right off the blocks. You typically see invariant characteristics persist through life. To me this is "the real you", the aspects of your personality that are with you throughout your life.

If I had to put a figure on it, to avoid argument I'd say it accounts for between 30 and 50% of your behaviour. The rest is shaped by social conditioning, life experiences and the environment we find ourselves living in. But even these could be said to be mediated in some part by the initial 30%.