lots of limitations, e.g. you cannot do very much experiments with human beings.

So, as we have faster and faster (cheaper and cheaper, more and more accurate) DNA sequencing + other things (chips/arrays, CHIP, structures etc), sample sizes in population studies are increasing, and molecular basis of many diseases come to our knowledge.

monitoring the "环境" of individual human is hard if not abs impossible. So, i see twin studies as just some medieval fantasies for old boys. Unless, you are extremely lucky, you wouldn't make much useful sense out of it.