A correlational study found that suicide is more likely to be the cause of death in high-altitude states, such as Colorado, than in lower-altitude states (D. Cheng, 2002). This relationship may arise for a number of reasons, such as decreased levels of oxygen from the higher altitude (McCook, 2002), the challenges of mountain living, differences in the age and education levels of the populations, or perhaps the fact that depressed people seek a type of solitude more often found in high-altitude states. One possible confound the study did not control for was the amount of time those who committed suicide had lived at the higher altitude: Those who were born in high-altitude states would have adapted to the lower oxygen level. Researchers could have investigated one possible explanation by examining this variable (McCook, 2002).