Serotonin signaling is vital for reward processing, and hence, also for decision-making. The serotonin transporter gene linked polymorphic region (5-HTTLPR) has been connected to decision making, suggesting that short-allele carriers (s) are more risk averse than long-allele homozygotes (ll). However, previous research has not identified if this occurs because s-carriers (i) are more sensitive to the uncertainty of the outcomes or (ii) are more sensitive to the magnitude of the outcomes. This issue was disentangled using a willingness-to-pay task, where the participants evaluated prospects involving certain gains, uncertain gains, and ambiguous gains. The results clearly favored the hypothesis that s-carriers react more to the magnitude of the outcomes. Self-reported measures of everyday risk-taking behavior also favored this hypothesis. We discuss how these results are in line with recent research on the serotonergic impact on reward processing.