Later in the 1990s, a modified dopamine hypothesis of schizophrenia was proposed (Davis, Kahn, Ko, & Davidson, 1991) based on, for example, neuronal lesions in the prefrontal cortex in rats resulting in increased levels of dopamine and in greater dopamine D2 receptor density in the striatum (Pycock, Kerwin, & Carter, 1980). This evidence concerns the gene DRD2 and schizophrenia.