Participants were four groups of volunteers hypothesized to be of increasing affective vulnerability as follows : nonsmokers lacking recurrent personal and familial history of MDD ( n = 20 ) , smokers lacking recurrent personal and familial history of MDD ( n = 21 ) , nonsmokers with history of recurrent personal and familial MDD ( n = 16 ) , and smokers with recurrent personal and familial history of MDD ( n = 16 ) .
