<P> Specifically, prenatal exposure to marijuana affects development of intelligence later in childhood, in a nonlinear fashion, with the degree of exposure . Heavy use by the mother within the first trimester is associated with lower verbal reasoning scores on the Stanford - Binet Intelligence Scale; heavy use during the second trimester is associated with deficits in composite, short - term memory as well as lower quantitative scores on the test; high exposure in third trimester associated with lower quantitative scores as well . A study by Fried and Smith indicated that marijuana exposure did not lead to a decrease in global intelligence but that it did lead to problems with executive functions in childhood . However, another study found that when influences such as maternal age, mother's personality and home environment, there was no longer a difference between children exposed to marijuana and those not exposed with relation to executive functions . </P> <P> Exposure to tobacco smoking has been associated with diminished intelligence and attentional problems . One study indicated that children whose mothers had smoked 10 or more cigarettes a day were between 3 and 5 months behind schoolmates in reading, maths and general ability . However, other studies have found no direct link between IQ and tobacco smoking with maternal intelligence accounting entirely for the relationship . </P> <P> There is also evidence that birth complications and other factors around the time of birth (perinatal) can have serious implications on intellectual development . For example, a prolonged period of time without access to oxygen during the delivery can lead to brain damage and mental retardation . Also, low birth weights have been linked to lower intelligence scores later in lives of the children . There are two reasons for low birth weight, either premature delivery or the infant's size is just lower than average for its gestational age; both contribute to intellectual deficits later in life . A meta analysis of low birth weight babies found that there is a significant relationship between low birth weight and impaired cognitive abilities; however, the relationship is small, and they concluded that, although it may not be relevant at an individual level, it may instead be relevant at a population level . Other studies have also found that the correlations are relatively small unless the weight is extremely low (less than 1,500 g)--in which case the effects on intellectual development are more severe and often result in mental retardation . </P> <P> It has been hypothesized that the development of genius in an area results from early environmental exposure to the topic in which the "genius" has prodigious knowledge or skill . This is utilizing the definition of genius that is not just a significantly higher than average IQ score, but also having some type of exceptional understanding or ability in a specific field . Einstein is often used as an example of genius; he did not demonstrate generalized exceptional intelligence as a child; however, there is evidence that he started exploring the ideas of physics and the universe at a young age . </P>

Identify biological and enviromental influences on development of the prenatal brain