To fund pro-positive or anti-negative externalities and more

The other day in my political economy of the middle east class we were talking about externalities. The professor asked us whether it was smarter to subsidize negative or positive externalities. I answer with negative and at the same time another students says positive. The conventional answer is that governments should subsidize positive externalities so that there is incentive for people to do good things. I then took the opportunity to rationalize my thought. Here is my argument: positive externalities have a built in or intrinsic value to the "offender" while negative externalities are usually driven by selfishness and a lack of care of anything other than the immediate interest of the "offender." In this sense it would make sense to subsidize or more accurately give an incentive for people perpetuating negative externalities to no longer partake whatever it is they are doing. Positive externalities will continue to be acted out based on the benefit that the "offender" and all of society gains. Obviously my point relies on the assumptions that people have feelings of benevolence and that the "offender" is aware of the externalities which they are putting forth. Although in a more strict sense of the term selfishness it would seem as though most negative externalities are perpetrated out of laziness or thoughtlessness than that of pure selfishness.

On a related note to this argument which I coincidently didn't even end up knowing about until I randomly stumbled across it on the internet last night, There is a BBC documentary by Adam Curtis called The Trap: What Happened To Our Dream Of Freedom. I have only watched the first episode but it is an interesting look at game theory applied to the real world. It is related in the sense of the politicians basis of action: selfishness. Ok, so it isn't exactly related by there is a psuedo-similar general topic there. And the video has a very interesting concept even if it is difficult to believe.