Twin studies are not worthless, but they are limited. If you are trying to figure out why people from the same city and class have differences in trait X, then

Twin studies are not worthless, but they are limited. If you are trying to figure out why people from the same city and class have differences in trait X, then a twin study will help find that out. But if you are trying to find out why people from different classes and/or locations have a difference in trait X, then a twin study will not really randomize the environment, because both twins will likely be adopted by someone of the same class.