It has been found that several nongenetic factors, such as age and education levels could influence cognitive function of healthy adults. Recently, investigators found that variances in COMT also have effects on cognitive functioning. However, this finding cannot be replicated by subsequent studies. Several reasons may contribute to the discrepancy. First, their sample sizes were too small. Second, they did not control for the nongenetic factors. The most recent study published on Cell in 2003 demonstrated that minor variation in brain-derived neurotrophic factor (BDNF) might also alter episodic memory in humans. The possible effects of this variation on other cognitive domains also require elucidation. Certainly, numerous other genes may influence cognitive functions too. The genes that modulate the activity of dopamine, serotonin, acetylcholine, adrenaline, noradrenaline, cannabinoid, estrogen, or glutamate deserve investigations at first.
