The Nature of Human Nature

I was recently asked by a fine young gentleman why I don't believe in 'human nature'. I wasn't surprised that this was a confusing assertion on my part, as the notion is a common one that many people share, even if they differ on what the term means.
Hence, I feel the need to explain myself. There are many things that I draw on, not all of which I can prove. Anyone is free to disagree with me, any of my premises and even the conclusion I draw from them, because I will be the first to say I could be completely off and I don't want anyone to think that I am totally confident in my rightness. Okay, disclaimer done.

If you went up to people on the street and said 'What is human nature?' or even 'What is human nature to do?', you would probably receive a variety of responses including some trite but witty catchphrases. When it comes to morality, this would be even stickier. Is it human nature to be good or to be selfish? Most people probably don't think about it. 'Look out for number one', yet at the same time 'People are inherently good' (we could argue what the definition of 'good' is, but that would be a whole separate argument).

This sets up my first problem.

1) There is no universal agreement on what is really 'human nature'. There are plenty of texts that you could draw on, such as the Bible, there are plenty of philosophers you could listen to and certainly one of them is probably right, but for the layman, it is still nearly impossible to say 'Human nature is to do X'. So anyone who justifies or claims 'human nature' as a premise has an already shaky argument because it is difficult to back up.

2) The misuse of an empty phrase.
My second issue is the Twinkie Defense. 'It's just human nature' is the modern day equivalent of 'The Devil made me do it'. It is an excuse for people to not have to take responsibility for their own actions based on genetics or some kind of innate characteristic that they have no control over. People may be predisposed to do certain things or be certain ways that would affect their behavior, I don't discount this. But many people who have the same maladies refuse to let it account for their actions and still exert self-control. Furthermore, those issues (say, a chemical imbalance) is a physical condition that is provable and quantifiable. 'Human nature' is an abstract idea based on a subjective perspective. It would never hold up in a court room, yet the phrase is bandied about every day.

Where this comes from:

One of the benefits of having three different teachers is you get three different approaches to history. One of my classes were asked 'Is it human nature to strive for freedom?', which is where I stated I did not believe in human nature as a response. And even if I did, the answer would be no.
This is where we get into the sticky 'could argue historical truth' part. My conclusions are based on the idea that I have an understanding of historical events and perspectives and as I was not there, I can't say this with 100% accuracy. You may view these same facts differently and with good reason. So bear in mind that I'm just going from what I know.
Here is an example: "Is it human nature to love your country and want what's best for it?' ie, 'Nationalism'. Well, this would be a problem considering that 'nations' as we understand them today didn't even exist until relatively recently. For example, during the Peloponnesian Wars, there was no firm idea of what it was to be 'Greek'. None of those individuals would have referred to themselves as 'Greeks'. It can be argued that there was some pride in being 'Roman' but even then, that had more to do with holding on to an individual.

Okay, back to the topic at hand.
Have people always strove for freedom?

This is a tough question. Another of my teachers would argue no, BECAUSE the idea of 'bettering yourself', being in a position other than where you are doesn't seem to have been put into practice until a little while ago, historically speaking. No one strove for freedom because it didn't occur to them (and those whom it might have were probably silenced out right). These people were told they were lesst han those in control over them and there was nothing they could do about it because it was their NATURE. See, there's that word again. Some innate characteristic of theirs made them inferior to these other people. This is the very premise of racism, that this natural trait is passed on to the next generation, without any real proof.
So those who were subjugated believed it was simply how things were, and that was that.
This didn't really change until we got this nationalism (and republicanism) thing going, because both are built on the notion of 'liberty, fraternity' (brotherhood) and equality', and it's really hard to back that up when you're not including everyone in the nation-state into the fold.

I hope this clears up the questions a bit, if not, feel free to post any questions and I'd be happy to clarify. This is still something I'm trying to solidify in my own mind, so questions are good because it helps me think about it in a new way.

Alright, fellow philosophers, have at thee.