GMAT考试写作指导：Argument范文四八

22.        The conclusion in this argument is that increased vigilance by drug enforcement
authorities has resulted in an increase in the illegal use of cocaine¹. The author reaches
this conclusion on the grounds that drug traffickers have responded to increased
enforcement efforts by switching from bulkier and riskier² drugs to cocaine. Presumably,
the author's reasoning is that the increased enforcement efforts inadvertently brought
about an increase in the supply of cocaine which, in turn, brought about the observed
increase in the illegal use of cocaine. This line of reasoning is problematic in two
important respects.
      In the first place, the author has engaged in "after this, therefore because of this"
reasoning. The only reason offered for believing that the increased vigilance caused the
increase in cocaine use is the fact that the former preceded the latter. No additional
evidence linking the two events is offered in the argument, thus leaving open the
possibility that the two events are not causally related but merely correlated. This in turn
leaves open the possibility that factors other than the one cited are responsible for the
increase in cocaine use.
      In the second place, the author assumes that an increase in the supply of cocaine is
sufficient to bring about an increase in its use. While this is a tempting³ assumption, it is
a problematic one. The presumption⁴ required to substantiate⁵ this view is that drug users
are not particular about which drugs they use, so that if marijuana and heroin⁶ are not
available, they will switch to whatever drug is available—cocaine in this case. The
assumption does not seem reasonable on its face. Marijuana, heroin, and cocaine are not
alike in their effects on users; nor are they alike in the manner in which they are
ingested or in their addictive⁷ properties. The view that drug users' choice of drugs is
simply a function of supply overlooks these important differences.
      In conclusion, the author has failed to establish a causal link between increased
enforcement efforts and the observed increase in illegal cocaine use. While the
enforcement activities may have been a contributing factor, to show a clear causal
connection the author must examine and rule out various other factors.