The current study will test the causal relationship between elevated levels of cortisol and the serotonin transporter gene (5-HTTLPR) as these factors influence sensitivity to environmental threat. The investigators predict that carriers of the short allele of the serotonin transporter gene who have elevated cortisol levels will be most sensitive to threatening environments, whereas carriers of the long allele who do not have elevated cortisol (placebo subjects) will be least sensitive.