The investigators propose to test each young adult twice; once on the lowest clinically relevant dose of MPH and once on a placebo (Vitamin C), with the order counterbalanced. The two testing sessions will be scheduled 4 weeks apart, once 90 minutes after taking 10mg MPH and once 90 minutes after taking a placebo (90 mg of Vitamin C). Peak plasma concentration of MPH is reached approximately 2 h after ingestion, thus to gain the maximum effect of MPH, the investigators will test subjects 90 minutes after ingestion. They will tell subjects that they are studying the effects of low-dose MPH on EFs, and that they expect it to help the EFs of some but hinder those of others, without telling them the study's predictions. Before each testing session, subjects will come to the lab 90 minutes in advance of the testing period. First, they will be given a pill they will take in front of a research assistant (MPH or placebo). The conditions will be counterbalanced across subjects within each subject group. This is a double-blind design, where neither subject nor tester will know which condition they will be in. The pills (10mg MPH, and 90mg Vitamin C) are made to be identical. They will be over-encapsulated with an opaque coloured capsule. The pills will be in kept in sealed containers that are labeled in such a way that neither the subjects, nor the testers, nor the PI, will not know which pill is which, until it is time for the data analysis. Only a pharmacist, at the compounding pharmacy, will know which container contains which pill.

At low doses the mode of action of MPH increases DA efflux specifically in PFC and preferentially enhances signal processing in PFC. Low dosages of MPH are often effective in improving EFs and specifically because of their effect on PFC. This slight increase of DA in the PFC is similar to the effects of mild stress on the brain, which is why the study uses a low dose of MPH as the pharmacological model of mild stress. Even mild stress markedly increases DA levels in PFC, impairing PFC function and EFs. The use of MPH is meant to mimic the effect of mild stress on the PFC and executive functions.

Purpose/objective: This double-blind study aims to compare performance on tasks of executive function between males, females when their estrogen levels are high, and females when their estrogen levels are low when they have undergone a pharmacological model of mild stress (low dosage of MPH) and when they have not undergone this stress. It also aims to compare the same across the three variants of the catechol-O-methyltransferase (COMT) genotype (methionine-methionine, methionine-valine, and valine-valine).

Hypotheses: MPH, like mild-stress, should raise PFC DA levels in COMT-Vals to optimal, but raise PFC DA levels in COMT-Mets past optimal (since low dose of MPH increases DA levels in PFC). While COMT-Mets generally show better EFs than COMT-Vals without MPH, on MPH that should be reversed with COMT-Vals performing better. An increase in PFC DA levels pharmacologically should mimic the sex difference in the effect of mild stress on EFs, harming females with high estrogen level's performance while enhancing males' performance on tasks of executive functions.