The current distinction between the terms sex difference versus gender difference has been criticized as misleading and counterproductive. These terms suggest that the behavior of an individual can be partitioned into separate biological and cultural factors. (However, behavioral differences between individuals can be statistically partitioned, as studied by behavioral genetics.) Instead, all behaviors are phenotypes—a complex interweaving of both nature and nurture.[41]
Diane Halpern, in her book Sex Differences in Cognitive Abilities, argued problems with sex vs. gender terminology: "I cannot argue (in this book) that nature and nurture are inseparable and then... use different terms to refer to each class of variables. The ...biological manifestations of sex are confounded with psychosocial variables.... The use of different terms to label these two types of contributions to human existence seemed inappropriate in light of the biopsychosocial position I have taken." She quotes Steven Pinker's summary of the problems with the terms sex and gender: "Part of it is a new prissiness -- many people today are as squeamish about sexual dimorphism as the Victorians were about sex. But part of it is a limitation of the English language. The word 'sex' refers ambiguously to copulation and to sexual dimorphism..."[42] Richard Lippa writes in Gender, Nature and Nurture that "Some researchers have argued that the word sex should be used to refer to (biological differences), whereas the word gender should be used to refer to (cultural differences). However, it is not at all clear the degree to which the differences between males and females are due to biological factors versus learned and cultural factors. Furthermore, indiscriminate use of the word gender tends to obscure the distinction between two different topics: (a) differences between males and females, and (b) individual differences in maleness and femaleness that occur within each sex."[43]
It has been suggested that more useful distinctions to make would be whether a behavioral difference between the sexes is first due to an evolved adaptation, then, if so, whether the adaptation is sexually dimorphic (different) or sexually monomorphic (the same in both sexes). The term sex difference could then be re-defined as between-sex differences that are manifestations of a sexually dimorphic adaptation (which is how many scientists use the term),[44][45] while the term gender difference could be re-defined as due to differential socialization between the sexes of a monomorphic adaptation or byproduct. For example, greater male propensity toward physical aggression and risk taking would be termed a "sex difference;" the generally longer head hair length of females would be termed a "gender difference."[46]