Question 51·Hard·Command of Evidence
Urban forestry programs are increasingly promoted as a low-cost way to improve public health. In a recent analysis of 40 U.S. cities, researchers reported that neighborhoods with at least 30% tree-canopy cover had childhood asthma rates 25% lower than neighborhoods with less than 10% canopy. Environmental epidemiologist Sana Malik, however, cautions that assigning the difference entirely to trees may be misleading. Her team reanalyzed the same dataset, adding controls for household income, traffic density, and access to primary care. Once these factors were included, the asthma gap nearly disappeared. This suggests that the apparent health advantage of tree-rich neighborhoods stems less from the trees themselves than from the socioeconomic conditions that typically accompany them.
Which choice, if true, would most directly support the claim in the boldface sentence?
For Command of Evidence questions asking which finding would support a claim, restate the claim as a simple cause-and-effect statement and identify what alternative explanation it’s pushing (here: socioeconomic factors rather than trees). Then choose the option that best tests the claim by isolating variables—ideally changing tree cover while holding income/traffic/access to care as constant as possible—and eliminate options that are irrelevant or that instead suggest the opposite causal story.
Hints
Clarify what the boldface sentence is arguing
Restate the boldface sentence in your own words: is it claiming that trees directly cause better health, or that something associated with tree-rich neighborhoods is really responsible?
Think about cause vs. correlation
The passage contrasts a correlation (more trees, less asthma) with an explanation that the correlation may be due to other factors (income, traffic, access to care). What kind of evidence would separate those explanations?
Look for a clean test
A strong supporting finding would change tree cover while holding socioeconomic conditions as constant as possible. Which option comes closest to that?
Eliminate choices that point to trees as the main cause
Cross out options that would make it seem like trees (or tree species) are directly driving asthma outcomes, since that would weaken—not support—the boldface claim.
Step-by-step Explanation
Understand the boldface claim
The boldface sentence argues that the real reason tree-rich neighborhoods have lower childhood asthma rates is not mainly the trees. Instead, it says the advantage comes from socioeconomic conditions that often accompany lots of trees (for example, higher income, less traffic, and better access to care).
Decide what strong supporting evidence would look like
To support this claim, we need evidence that helps separate the effect of trees from the effect of socioeconomic conditions.
Strong support would come from a scenario where:
- tree cover increases substantially, but
- key socioeconomic conditions (income, traffic, access to care) do not meaningfully improve, and
- asthma rates do not improve.
That outcome would suggest trees alone are not driving the health advantage.
Evaluate the choices for directness
Check each option for whether it tests the key issue: Are trees themselves causing lower asthma rates, or are they just associated with better conditions?
- A result showing asthma improves right where trees are added (without addressing other conditions) tends to suggest a tree effect, not a socioeconomic-confound explanation.
- A result focusing on pollen/allergen differences points to tree characteristics as a driver, which does not support the claim that trees are not the main factor.
- A result in affluent areas with no change over time is weak because it doesn’t isolate what happens when trees change while conditions stay comparable.
Select the option that best isolates trees from socioeconomic factors
The best support will be the option where tree canopy increases in low-income areas (so socioeconomic conditions are less likely to have improved in tandem), and yet asthma does not decrease—indicating that adding trees alone does not create the health advantage.
State the supported conclusion and identify the correct choice
The finding that most directly supports the boldface claim is:
A midsize city that dramatically increased tree-canopy cover in its lowest-income neighborhoods saw no subsequent decline in childhood asthma rates.
This supports the idea that the apparent advantage of tree-rich neighborhoods likely comes from accompanying socioeconomic conditions rather than the trees themselves.