Compared to controls, sons of alcoholics have been reported to exhibit decreased sensitivity to a challenge dose of alcohol (e.g. a smaller increase in body-sway: Schuckit, 1985). We reanalyze alcohol challenge data from a general population sample of Australian twin pairs (Martin et al., 1985a, b). By fitting direction-of-causation models, we show that in males differences in body-sway are largely a consequence, not a cause, of differences in alcohol consumption patterns. In females, direction-of-causation could not be resolved.