Depends on measure, genes play as much a part as environment. In less than ideal upbringing a lot of mental resources are used just for survival, as well as sometimes there being a social stigma placed on being well educated, sometimes this is dangerous even. In a good home environment little resources are devoted to survival and concentration on academic work can be a lot greater leading to better scores.
I would say people are a lot smarter than they think.
If they will just put some thought into it.
Take two brothers 1 is of high IQ and is a top class surgeon, the other is of average IQ and a world class musician .. how do you rate intelligence, everyone has a talent for something ..
Because some people are more intelligent than others. :p
I think so, but it is also dependant on upbringing and luck.