[grandpa]

You could test this by looking at whether pairs of fraternal twins are as similar on personality traits as pairs of identical twins. If they are more different, then genes are the cause.

[rusk]

There's been lots of these twin studies. Most of what we know about genetic influences has been determined this way.

https://en.wikipedia.org/wiki/Twin_study

[callesgg]

You cant objectively measure the difference between two people.