The biochemistry of a drug is only one of a wide range of factors that interact to affect the individual who takes the drug. Reactions to alcohol and other sedative drugs seem to depend upon whether the drinker is an introvert or an extrovert. Introverts seem to be comparatively resistant to the effects of alcohol, whereas extroverts succumb to its intoxicating influence much more readily. The opposite is true of stimulant drugs like amphetamine or caffeine; introverts react more strongly to these drugs and extroverts less. The effect of a drug is also influenced by the takers' beliefs, attitudes and expectations, by what he or she have been told by others, and by the immediate social circumstances in which the drug is taken. Because psychological factors play a large part in determining how people respond to drugs, it is possible for them to react as if they had taken a drug even when no such drug has been used, i.e., the placebo effect. There are cases in which people have become addicted to placebos, a convincing illustration of the essentially psychological nature of drug dependence. Figure