Our textbook describes four influences that have been associated with reduced me

Risk factors associated with reduced mental ability are: 
Poor prenatal care (including maternal drug use during pregnancy);
Malnutrition;
Exposure to toxins such as lead; and
Stressful family circumstances such as having a father who does not live with the family, a mother with a history of mental illness, parents with limited work skills, and a history of stressful events such as domestic violence early in life.
A healthy and stimulating environment can raise mental performance.
In the Abecedarian Project, inner-city children who got lots of mental enrichment at home and in child care or school, starting in infancy, had much better school achievement than did children in a control group.