Reading Comprehension (argument) — GRE Practice
Recent studies have suggested that urban trees provide significant psychological benefits to city residents. People who live near trees report lower stress levels, greater life satisfaction, and faster recovery from mental fatigue than those in tree-scarce areas. Some researchers attribute these effects to the restorative properties of natural environments: exposure to greenery prompts involuntary attention — a passive, effortless form of focus — that allows the directed attention system to replenish itself. Others maintain that the benefits may stem primarily from social factors, since neighborhoods with more trees tend to have stronger community bonds and lower crime rates, both of which independently improve mental well-being.
The passage suggests that researchers disagree about which of the following?