This chapter discusses the convergent and discriminant validity of different measures of social mastery motivation in relation to object mastery, attachment and temperament. It looks at the relation of social mastery motivation to measures of object mastery, attachment, sociability and difficult temperament. Significant positive correlations between the three measures of social mastery motivation with those of attachment, temperament and object mastery would be evidence against the discriminant validity of existing measures of social mastery motivation. However, to assess attachment security, they were compared to the criterion sort for security of attachment rather than to the criterion sort for social mastery motivation. The chapter assess the independence of social mastery behaviours from the possibly related but theoretically independent constructs of object mastery, attachment security and temperament. It presents results support the conceptual validity of the construct of social mastery motivation; they also highlight problems in regard to the measurement of this construct.