Happiness is surprisingly unmoved by external factors (Lykken & Tellegen 1996), because the genetics accounts for about 50% of the variance in happiness (Lyubomirsky et al. 2005; Stubbe et al. 2005).

Caution: heritability, as in the statistical concept, is defined in a way that has some rather counter-intuitive implications. One might think that if happiness is 50% heritable, then happiness must be 50% "hardwired". This is incorrect, and in fact the concept of heritability is theoretically incapable of making such a claim. (I'm not saying lukeprog made this mistake, but someone is likely to make it.)

The definition of heritability is straightforward enough: the amount of genetic variance in a trait, divided by the overall variance in the trait. Now, nearly all humans are born with two feet, so you might expect the trait of "having two feet" to have 100% heritability. In fact, it has close to 0% heritability! This is because the vast majority of people who have lost their feet have done so because of accidents or other environmental factors, not due to a gene for one-footedness. So nearly all of the variance in the amount of feet in humans is caused by environmental factors, making the heritability zero.

Another example is that if we have a trait that is strongly affected by the environment, but we manage to make the environment more uniform, then the heritability of the trait goes up. For instance, both childhood nutrition and genetics have a strong effect on a person's height. In today's society, we have relatively good social security nets helping give most kids at least a basic level of nutrition, a basic level which may not have been available for everyone in the past. So in the past there was more environmental variance involved in determining a person's height. Therefore the trait "height" may have been less hereditary in the past than now.

The heritability of some trait is always defined in relation to some specific population in some specific environment. There's no such thing as an "overall" heritability, valid in any environment. The heritability of a trait does not tell us whether that trait can be affected by outside interventions.