Question

Children whose biological parents both have Tic Syndrome Z (TSZ), which is characterized by the involuntary contraction of certain muscles, are about four times more likely to develop such contractions than are children whose biological parents do not have TSZ. It is likely, therefore, that predisposition to TSZ is an inherited trait.
Which of the following, if true, would most strengthen the conclusion above?

• A. Children whose parents have TSZ are more likely to develop TSZ if they are under unusual stress at school or at home than if they are not under such stress.
• B. Children whose biological parents do not have TSZ are more likely to develop TSZ if they are raised by adoptive parents with TSZ than if they are raised by their biological parents.
• C. Children whose biological parents have TSZ are as likely to develop TSZ if they are raised by adoptive parents who do not have TSZ as if they are raised by their biological parents.
• D. Children whose biological parents have TSZ and who develop TSZ usually avoid developing a severe form of the syndrome if they seek treatment for TSZ shortly after developing the first signs of it.
• E. Children with TSZ whose biological parents do not have TSZ are less likely to have the syndrome diagnosed when symptoms first appear than are children with TSZ whose biological parents have TSZ.

Hint:

For any "nature vs. nurture" argument, the gold standard for strengthening or weakening is an adoption study. To strengthen the "nature" (genetic) argument, show that adopted children are more like their biological parents than their adoptive parents. To strengthen the "nurture" (environment) argument, show the reverse.

Answer 1

The Correct Option is C

Step 1: Understanding the Concept:
The argument concludes that a trait (predisposition to TSZ) is inherited (genetic) based on a correlation between parents and their biological children. The classic challenge to this type of argument is the "nature vs. nurture" debate. The correlation could be due to shared genes (nature) or a shared environment and upbringing (nurture). To strengthen the "nature" conclusion, we must weaken the "nurture" alternative.
Step 2: Key Formula or Approach:
The best way to separate genetic factors from environmental ones is to use an adoption study. We need to find an answer choice that shows that the environment (who raises the child) doesn't make a difference, while the genetics (who the biological parents are) does.
Step 3: Detailed Explanation:
- The argument is: Biological parents with TSZ have children with TSZ \(\rightarrow\) TSZ is genetic.
- The weakness is: Maybe the parents with TSZ create a home environment that causes their children to develop TSZ (e.g., through stress or learned behavior).
- (C) This option directly addresses and eliminates the "nurture" explanation. It takes a group of children with a genetic predisposition (their biological parents have TSZ) and compares two subgroups: one raised by their biological parents (genetics + TSZ environment) and one raised by adoptive parents without TSZ (genetics + non-TSZ environment). The finding that both groups are "as likely to develop TSZ" shows that the environment they were raised in didn't matter. The only constant factor is their genetics. This provides powerful evidence that the trait is inherited.
- (A) This suggests an environmental trigger (stress), which would add a layer to the nurture side of the argument, not strengthen the genetic claim.
- (B) This would weaken the argument. It shows that children without the genetic predisposition are more likely to get TSZ if raised in a TSZ environment, suggesting that nurture is a powerful factor.
- (D) and (E) discuss treatment and diagnosis, which are irrelevant to the cause of the predisposition itself.
Step 4: Final Answer:
By showing that the environment a child is raised in has no effect on their likelihood of developing TSZ, this option isolates genetics as the causal factor, thus strongly strengthening the conclusion.