Psychologists conducted a series of experiments to test the effect upon schoolchildren of violence in films. In the first experiment, grammar school children were shown a film that included scenes of a male teenager engaging in violent acts against others, such as punching, pushing, and kicking. During a free-play session following the film viewing, 42 percent of the children were observed to engage in one or more violent acts similar to those in the film. In a second experiment, a different group of children was shown a similar film featuring a female teenager. Only 14 percent of the children were observed behaving violently afterward. The psychologists concluded that children are more likely to imitate violent behavior on film when a male model is shown than when a female model is shown.
Which of the following, if true, would most seriously weaken the psychologists’ conclusion?
(A) In both experiments, the victims of the filmed violence included both males and females.
(B) In the second experiment, 28 percent of the children appeared upset during the viewing the violent film scenes.
(C) The first group included 19 male students and 20 female students; the second group included 20 male students and 21 female students.
(D) In the first group, 58 percent of the children appeared bored during the showing of the film, and 12 percent fell asleep.
(E) The percentage of children known to have discipline problems prior to the experiment was greater in the first group than in the second group.
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another vote for (e) by POE

one more vote for E. Here's my line of reasoning.

This stimulus falls into a common reasoning pattern in which the author concludes that because two groups of people, one that is supplied with a product whereas the other is not, present differences between them. The author attributes this differences to the product but does not consider that the differences might have already been there before the supply.

As a analogy consider a product that claims to make people loose weight. Consider two groups of people. People in group 1 weight less than people in group 2. People in group 1 take the product for 6 weeks after of which they weight less than people in group 2. The study concludes that the product caused that lost in weight. This is clearly flawed.

Regarding the stimulus

A) In both experiments, the victims of the filmed violence included both males and females.
This answer address the victims of the filmed that is not discussed in the stimulus. out
B) In the second experiment, 28 percent of the children appeared upset during the viewing the violent film scenes.
For this to weaken the argument you have to assume that when people view a film while they are upset the film does not influence them. Since we cannot make that assumption, we can rule B out.

(C) The first group included 19 male students and 20 female students; the second group included 20 male students and 21 female students.
The stimulus talks about percentages so raw numbers are useless
D) In the first group, 58 percent of the children appeared bored during the showing of the film, and 12 percent fell asleep.
That the children did not like the film does not weaken that the film influenced them.
(E) The percentage of children known to have discipline problems prior to the experiment was greater in the first group than in the second group.
This states that the groups were different prior to the experiment. Correct

E

The expriment was not good because the two groups were not alike.

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good explanations

The point is that E implies that the 2 groups are different so the conclusion can not be made or that the discipline problems of the first groups also imply more violence among that group so is logical to conclude that group to be more violent?
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discipline problems-->more violence, for me is assuming to much
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the initial contents of both groups were not equal, i.e. the first group has had health problems which does effect results of the expetiment.
Hence: E