In this article I flesh out support for observations that scientific accounts of social groups can influence the very groups and mental phenomena under investigation. The controversial hypothesis that there are hardwired differences between the brains of males and females that contribute to sex differences in gender-typed behaviour is common in both the scientific and popular media. Here I present evidence that such claims, quite independently of their scientific validity, have scope to sustain the very sex differences they seek to explain. I argue that, while further research is required, such claims can have self-fulfilling effects via their influence on social perception, behaviour and attitudes. The real effects of the products of scientists' research on our minds and society, together with the fact that all scientific hypotheses are subject to dispute and disconfirmation, point to a need for scientists to consider the ethical implications of their work.