Homer is "a (happy) slave to his various appetites". He has an apparently vacuous mind, but occasionally exhibits a surprising depth of knowledge about various subjects, such as the composition of the Supreme Court of the United States, Inca mythology, bankruptcy law, and cell biology. Homer's brief periods of intelligence are overshadowed, however, by much longer and consistent periods of ignorance, forgetfulness, and stupidity. Homer has a low IQ of 55, which would actually make him unable to speak or perform basic tasks, and has variously been attributed to the hereditary "Simpson Gene" (which eventually causes every male member of the family to become incredibly stupid), his alcohol problem, exposure to radioactive waste, repetitive cranial trauma, and a crayon lodged in the frontal lobe of his brain. In the 2001 episode "HOMR", Homer has the crayon removed, boosting his IQ to 105; although he bonds with Lisa, his newfound capacity for understanding and reason makes him unhappy, and he has the crayon reinserted. Homer often debates with his own mind, expressed in voiceover. His mind has a tendency to offer dubious advice, which occasionally helps him make the right decision, but often fails spectacularly. His mind has even become completely frustrated and, through sound effects, walked out on Homer. These exchanges were often introduced because they filled time and were easy for the animators to work on. They were phased out after the producers "used every possible permutation".