Dan Fessler in his blog over at the International Cognition and Culture Institute (h/t: Dan Sperber):
Lesson 5: Understanding human diversity is vital to generating and testing hypotheses about the mind. Read ethnographies describing other cultures, travel whenever the opportunity presents itself, and try to surround yourself with colleagues who are familiar with disparate ways of life – what seems self-evidently true about humans to you may not seem so to others. If there is reason to suspect that the features of the mind at issue could be variable across groups, whenever possible, employ samples that vary along potentially relevant dimensions.
A second reason to attend to human diversity is that many adaptations can be expected to be facultatively adjusted or deployed. In order to test for such possibilities, or even to recognize that they might exist, we must consider the range of physical and social ecologies that humans inhabit. Consider, for example, the case of sex differences in reactions to sexual versus emotional infidelity. In their pivotal paper on the subject, Buss et al. (1992) insightfully noted that this effect should vary across cultures as a function of the degree of male parental investment. However, although much research has now been done on jealousy, some in cultures other than that of the original authors, nonetheless, investigators have yet to compare results across samples selected specifically with regard to variation along this critical dimension. A preliminary effort in this regard (Yamashita, 2005) suggests that the Na (Mosuo) culture of Southwest China could provide an ideal setting for such an investigation. However, as this example suggests, testing hypotheses across ecologies that differ along functionally relevant axes will not always be easy.