The major source of prospective data on low LR comes from the 35-year-long San Diego Prospective Study (SDPS) with single dose oral alcohol challenges in 453 drinking men (usual past consumption three drinks per occasion) at about age 22 (range 18–25). Half of participants (probands) had an alcohol dependent father and half had no relatives with AUDs, with the two groups matched on age, race, education, drinking and other substance use histories (Schuckit and Gold, 1988; Schuckit et al., 2000). Over time, SRE data were gathered from drinking spouses and offspring, generating information on ~1620 individuals. Relationships of SRE-based low LR to heavier drinking and future alcohol problems were also prospectively documented in the Avon Longitudinal Study of Parents and Children (ALSPAC), the Collaborative Study on the Genetics of Alcoholism (COGA), and other investigations (e.g., Daeppan et al., 2000; Schuckit et al., 2001, 2008). Sibling-pair, twin, and family studies indicate low LR heritabilities of 40%–60% (e.g., Heath et al., 1999; Joslyn et al., 2008; Kalu et al., 2012; Schuckit et al., 2001; Vicken et al., 2003).
