Scientist: My research indicates that children who engage in impulsive behavior similar to adult thrill-seeking behavior are twice as likely as other children to have a gene variant that increases sensitivity to dopamine. From this, I conclude that there is a causal relationship between this gene variant and an inclination toward thrill-seeking behavior. Which one of the following, if true, most calls into question the scientist's argument? ['Children are often described by adults as engaging in thrill-seeking behavior simply because they act impulsively.', 'Many impulsive adults are not unusually sensitive to dopamine.', 'Many people exhibit behavioral tendencies as adults that they did not exhibit as children.', 'It is not possible to reliably distinguish impulsive behavior from other behavior.']