Much of experimental design in psychology is an effort to attenuate the effect of individual differences so that you can find the common (and probably core) cog

Much of experimental design in psychology is an effort to attenuate the effect of individual differences so that you can find the common (and probably core) cognitive processes. Sometimes the risk to external validity is worth it in order to reduce the complexity of data and models from confounding variables. So we work with a system where most science is conducted with a relatively homogeneous sample of college undergraduates who are convenient to obtain, while well replicated or foundational findings are later tested across cultures to determine degree of generalization. It seems an ok compromise to me.