Criminologist: Some legislators advocate mandating a sentence of life in prison for anyone who, having twice served sentences for serious crimes, is subsequently convicted of a third serious crime. These legislators argue that such a policy would reduce crime dramatically, since it would take people with a proven tendency to commit crimes off the streets permanently. What this reasoning overlooks, however, is that people old enough to have served two prison sentences for serious crimes rarely commit more than one subsequent crime. Filling our prisons with such individuals would have exactly the opposite of the desired effect, since it would limit our ability to incarcerate younger criminals, who commit a far greater proportion of serious crimes.
In the argument as a whole, the two boldfaced portions play which of the following roles?
逻辑链:一些立法者认为把已经两次犯罪的人永远关在牢里——可以降低犯罪率(作者要反驳)——因为他会把有可能犯罪的人永远关起来——但是老到能够犯两次罪的人很少会再犯罪了——把我们的监狱塞满这样的人会得到反效果(论述的主要结论)——这会限制我们关押有能力继续犯罪的年轻罪犯的能力