Three personality types, labeled resilient, over-controlled, and under-controlled, were identified through cluster analysis of classroom observations of 63 children, and used to understand biological, cognitive, and behavioural processes that influence academic achievement and aggression. Resilient children were found to be high in trait cortisol and high in academic achievement. Under-controlled and over-controlled children showed the greatest change in cortisol levels under stress, low levels of academic achievement, and attributed hostility to others in ambiguous situations. Under-controlled children also exhibited high levels of externalizing behaviour in the classroom. The findings suggest that the single processes or traits assessed in this study do not mediate the associations of personality types to academic achievement and behaviour. The implications of the findings for the personality type construct and for personality processes are discussed. Copyright © 2005 John Wiley & Sons, Ltd.