George Hutcheon writes: "Pokémon's fourth generation brought us a number of additions to gameplay. Alongside major advances like global online trading and the physical/special split, the games also introduced more subtle developments, like gender differences. In nature, animals of opposite genders frequently look quite different. This is called sexual dimorphism, and I was very glad to see this incorporated into the games. There was one problem, though: previous generations had depicted different genders as identical to each other (with the exception of Nidoran), so it seems Game Freak were reluctant to make these new differences too obvious for fear of contradicting earlier games. As a result, the Pokémon games' equivalent of sexual dimorphism is expressed via quite subtle, often barely noticeable differences."