For the last 140 years, scholars have been preoccupied with identifying the exact factors that influence one's IQ. The numerous studies on the subject have led most scientists nowadays to the belief that one's IQ is determined by a variety of both genetic and environmental factors, although there is contention about the exact weight of each.

The majority of studies on intelligence have shown that environmental factors account for about 25% of the differences in people's IQ scores. The factors that have been of greatest interest to scholars include prenatal development, nutrition, birth order, home and family environment, and the effects of schooling.

Prenatal development

Scientists have discovered many factors during a woman's pregnancy that could affect a child's cognitive development. Among them are:

… the mother's health, including her nutrition and smoking and drinking habits during pregnancy, her age and the number of previous pregnancies, the interval since her last pregnancy, blood type and Rh incompatibility of mother and fetus, her history of X-ray exposure, and her red blood cell count, to list a few (Jensen; 169).

Nutrition

Despite researchers' initial belief that nutrition played a significant role in a child's cognitive development, so far, there has been no substantial evidence that differences in nutritional habits have a noticeable effect on IQ, unless a child has suffered severe and prolonged malnutrition in early childhood.

Birth order

Surprisingly, statistical data shows that birth order can somewhat affect IQ:

Each successive child born into a family has, on average, a slightly lower IQ, by about .7 IQ point, than the previous born child… A parallel effect is found on scholastic achievement as well (Jensen; 171).

Home and family environment

One of the factors that many people attribute an important role to in mental development and differences in IQ is home and family environment. The latter encompasses variables such as:

… neighborhood, the number of rooms in the home ... the number of magazines and books, the parents' educational and occupational level, family income, whether private music lessons and dance lessons are given to child, membership in established organizations, and travel experiences (Jensen; 173).

Schooling

In addition, scholars have also examined the effects of schooling on a child's IQ. They have discovered that differences in school quality have no detectable influence on a child's intelligence test scores. What proves to be of greater significance is regular school attendance, especially in elementary school.

Genetic factors

Besides environmental influences, most scientists today believe that genes also play a part in determining one's IQ. However, there is currently an intense debate whether genetic or environmental factors are the deciding influence.

To sum up, science has come a long way in understanding the different factors that shape one's intelligence. However, there are still contentious issues that need to be clarified.

References:

Jensen, Arthur R. Straight Talk About Mental Tests. New York: The Free Press, 1981. 168-190.