Brains and intelligence vary as much as humans do. Intellectual abilities can rise, fall, zigzag, or stay the same, depending on genes and on the specifics of each life. Intelligence is multi-directional, multi-cultural, multi-contextual, and plastic. Generally, brain functioning is maintained: if you think deep and clearly at age 20 you will probably do so at age 60. One leading theoretician, Charles Spearman (1927), proposed that there is a single entity that he called general intelligence (g), which each adult has to some degree. Even though g cannot be measured directly, it can be inferred from various abilities, such as memory, reasoning,and vocabulary. Measuring those abilities produces an IQ score. That score correlates with healthand wealth in adulthood. Once a person reaches adulthood, an IQ score indicates whether that adult is a genius, average, or slow, no matter what the person’s age. In childhood, a person’s score depends not only on the items correct, with different items for preschoolers and other children, but also on the child’s age. That, however, is not true for adults. The same IQ tests are usually given to people aged 18 to 88, and they are scored the same way. The belief that g exists still influences thinking and testing on intelligence. Many neuroscientists seek genetic underpinnings for the intellect. Effort to find specific gene or abilities compromise g have not succeeded (Deary et al., 2010; Haier et al., 2009), but some aspects of brain function, particularlyin the prefrontal cortex, hold promise (Barbey et al., 2013; Roca et al., 2010). Other scientists believe that g may arise from prenatal brain development, experiences in infancy, or physical health. They emphasize the role of experience, noting that national and ethnic difference in average IQ scores suggests that context is crucial. IQ scores typically increase, or are at least maintained. Nature and nurture interact- and adults are smarter because of it.