In Morel's theory, degeneration was synonymous with anything that was different from the natural or normal state. These abnormalities were caused by environmental influences like diet, disease, and moral depravities or traits that were passed from generation to generation like alcoholism and living in the slums. Due to the law of progressivity, these degenerations would get worse in each generation to produce more criminals and neurotics with worse degenerations. Over time, the degenerations would progress until later generations (specifically the fourth generation) were so idiotic that they were essentially sterile and the abnormal family would die out. This theory explained why there was an increase in mental disorders and also allowed Morel to relate very different diseases as caused by previous generations because they had become more variable over time. Since there was an increase in mental disorders, Morel believed that society was approaching extinction of the imbeciles. He believed that the most degenerative illness was insanity. Morel was able to categorize degenerations into four main categories: hysteria, moral insanity, imbeciles, and idiots.