Critical Reasoning

"Life expectancy" is the average age at death of the entire live-born population. In the middle of the nineteenth century, life expectancy in North America was 40 years, whereas now it is nearly 80 years. Thus, in those days, people must have been considered old at an age that we now consider the prime of life.

Which of the following, if true, undermines the argument above?

(A) In the middle of the nineteenth century, the population of North America was significantly smaller than it is today.

(B) Most of the gains in life expectancy in the last 150 years have come from reductions in the number of infants who die in their first year of life.

(C) Many of the people who live to an advanced age today do so only because of medical technology that was unknown in the nineteenth century.

(D) The proportion of people who die in their seventies is significantly smaller today than is the proportion of people who die in their eighties.

(E) More people in the middle of the nineteenth century engaged regularly in vigorous physical activity than do so today.

The right answer should give us reason to believe that even though avg age at death was 40, people were not necessarily considered old at that age. This could happen if a lot of people died "before their time". You can imagine a society in which when you reach the prime of life, you have to go off to war and die. In such a society, life expectancy would be low, but people would not be considered old at death-age.

The answer is B. I go through the question in detail in the full solution below (taken from the GMATFix App).



-Patrick

How I'd do it:

To weaken an argument, you want to argue FOR the OPPOSITE of the original conclusion. If their argument concludes that "People used to be considered old at an age that we now consider the prime of life", then we want to argue for the opposite: "People were NOT considered old at 40; old age has always represented about the same number of years."

(A) doesn't help: the population size is irrelevant; we only care about the relative ages found in that population.

(B) is great. Life expectancy is now so much higher because we don't have lots of infant mortality. Think mathematically about why this would be so bad: if we have lots of people dying at age 0, then the average age of death has lots of 0s in its sum ... which would REALLY bring down life expectancy. But this also implies that people who don't die as infants live to about the same age as do people today ... so old age has always represented about the same number of years. Great!

(C) is irrelevant: we don't care WHY old people today got to be old, only whether old people in the old days were the same age as old people are today.

(D) doesn't tell us about the olden days, only today, so we still have no way of comparing.

(E) is random and useless.