The natural aging process explains why IQ scores differ among different ages of people. When people are younger, their brains are fresher. Once they grow beyond infancy, perception and comprehension of surroundings becomes sharper and quicker, and then peaks in adulthood. After this peak, senses dull and IQ scores suffer as a result. The Mandel School study tested both elderly persons, with an average age of 70, and college students, with an average age of 20, and demonstrated how the poorer perception typical to old age affects IQ scores. The college students always tested better than their elderly counterparts, except when the tests were doctored so that the students' perception was altered to replicate perception typical in old age.
A person's age compared to ages of siblings may also affect IQ scores. A 1980 report titled, "Birth Order, Age-Spacing, IQ Differences and Family Relations," noted repeated observations that firstborns, such as among eminent British scientists, tend to be more intelligent than younger siblings. While some remain skeptical, others theorize that since firstborns, because they're the oldest, often find themselves in positions of leadership, they become more task-oriented and more prone to intellectual achievement. Younger siblings tend toward preoccupation with sociability and peer relationships.
IQ scores vary not just among siblings, but also among generations. A phenomenon known as the Flynn Effect, attributed to political studies professor James Flynn, shows how IQ scores in the U.S. increase by three to four points per decade. So American adults born in 1990 should be three to four IQ points smarter than those born in 1980. Different rates of increase by decade have been observed in other countries as well. The reason, says Flynn, isn't genetic, but environmental. He believes that better nutrition and smaller families, in which children receive more focused attention from parents, are among the biggest factors.
The influence home environment has on IQ was further emphasized by research done by Arnold Sameroff. His and others' studies showed how environmental risk factors experienced by four-year-olds negatively affected IQ. For example, four-year-olds with an emotionally cool mother scored almost 20 IQ points lower than those whose mothers expressed positive emotions toward them. Another study showed that gifted children, those with IQs of 140 or more, can potentially lose intellectual ability over time if not challenged according to their intellect. Without challenging education, they become bored with school and susceptible to an underachieving lifestyle.