 But part of it is a limitation of the English language.The word 'sex' refers ambiguously to copulation and to sexual dimorphism..." 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.