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Most researchers agree that genetic factors account for about 40-80% of the variation in IQ test results.  If this is true, then environmental or other factors must account for 20-60% - which is a large range. 

Some environmental factors that have a large effect upon later IQ test results include prenatal ones, such as whether or not the birth mother drank large amounts of alcohol during her pregnancy (fetal alcohol syndrome) or was exposed to large amounts of lead.  Factors that have an effect during the early years include schooling, the quality of toys used, and the amount of external stimulus the child was subjected to.   Prolonged malnutrition also has a marked negative effect on IQ test results.  In contrast, babies who were breastfed have IQs up to 10 points on average higher than those who weren’t by the age of three. 

In spite of what many may believe, there is only a slight correlation between birth order and family size and IQ test results.  Likewise, there are no discernable differences in IQ between the sexes. 

A very large factor in determining IQ test results is the level and quality of schooling a child receives. The Head Start program in the United States is a federally funded preschool program for children from low income families.  Head Start provides children with activities that might enhance cognitive development, including reading books, learning the alphabet and numbers, learning the names of colors, drawing, and other activities. These programs often have large initial effects on IQ test results and children who participate gain as much as 15 IQ points compared to control groups of similar children not in the program. The educational correlation for IQ test results continues into adulthood, with college graduates typically scoring higher than non college graduates. 

IQ test results also vary based on immediate factors, such as diet, mood and time of day. It has been proven that they can vary up to 15 points based on a wide variety of immediate environmental factors.  An analysis of 1 million students in a New York school district showed that school cafeteria food affected IQ scores to an astonishing degree.   When preservatives, coloring, dyes, and artificial flavors were removed from the cafeteria, researchers found that 70,000 students performed two or more IQ grade levels higher than before.