See Robert Plomin's book, Blueprint for a great read on the topic.

Basically, for any complex trait, including personality traits, it has been shown over and over that about 50% of the variance between people is attributable to the genes. The other 50% is attributable to environment. But that 50% of the variance that is attributable to environment is non-systemic meaning the results are completely unpredictable and unstable. Meaning the outcome is completely unpredictable and also doesn't have the same outcome at any given period of time.

To drive the point home:

• Identical twins raised in separate households are consistently shown to be about 50% similar in all complex traits, from obesity to aggressive behavior to intelligence.

• Adopted, unrelated children raised together have approximately zero similarity with one another on how they turn out to be, and zero correlation in complex traits with the family that raised them.

• Any "environmental" effects (like how the kid was raised) are apparently so extremely complex and unstable that there are simply no correlations in how they turn out. Of course there are environmental effects, but they are not the result of anything we have yet been able to correlate to anything.