...studies from twins

Twin studies are an invaluable tool in the study of heritable traits.  Identical twins are, for all intents and purposes, genetic clones, sharing 100% of genetic material.  Non-identical or fraternal twins however, share 50% of genetic material, the same amount you would share with any of your siblings.  So one could compare any trait that conceivably has a heritable element, such as, for example, foot size, hair colour, height, or weight, and measure the variability of such traits between identical and fraternal twins.  These differences can be used to calculate the ‘heritability’ of a specific trait, which is the proportion of the variability that can be explained by genetics, as opposed to non-genetic reasons such as the environment; or to put it simply, to consider the influence of ‘nature vs nuture’.