Genetics
The way the studies are done is like this. Take a group of identical twins and a group of fraternal twins. Measure them for various traits. Do a statistical analysis (such as analysis of variance). This tells you to what extent the trait is inherited. Those traits which are partly inherited will be significantly more similar in identical twins. Studies like this may be carried further, by comparing identical twins brought up together with identical twins brought up in different circumstances. That gives a handle on how much circumstances can alter the outcomes of genetically identical people.