Although instrumental avoidance learning has been the subject of numerous investigations and extensive theoretical discussion (e.g., Bolles, 1971; Hermstein, 1969; Rescorla & Solomon, 1967; Seligman, 1970; Solomon, 1964, 1980; Solomon & Wynne, 1954), in recent years attention has focused on phenomena associated with avoidance behavior, such as learned helplessness (e.g., Alloy & Seligman, 1979; Maier & Jackson, 1979; Maier & Seligman, 1976) and Pavlovian-instrumental interactions (e.g., Overmier & Lawry, 1979). This trend has certainly advanced our understanding of the role of Pavlovian processes in avoidance behavior, but it has left unanswered fundamental questions about the psychobiological mechanisms that underlie the avoidance learning process. One approach to this issue is the analysis of the genetic contributions to individual differences in avoidance learning, that is, the determination of what predisposes an individual animal to learn to avoid.