The chapter starts by giving an overview on how intelligence has been defined over the years. First it was thought to be only a sensory capacity, then merely abstract thinking. The most current idea is that we each possess varying degrees of intelligence in different areas. For example, some people are high in linguistic intelligence, while others are high in spatial intelligence. IQ testing is the most common way to test intelligence despite several shortcomings such as the eugenics movement that sterilized 66,000 North Americans in the early part of the 20th century. The reliability of IQ tests is then brought into question and how consistent an IQ score is over time. Average IQ scores for various demographics are compared to each other in an attempt to find the causes of high and low IQ's.