Study: Green space & Wellbeing
A growing base of research suggests a positive association between green space in urban communities and improved mental health (Lee & Maheswaran, 2011). Conceptually, many would agree that public green space provides open areas for physical activity, social engagement, and stress reduction, yet many communities have not made rigorous community planning efforts (Fan et al., 2011). A recent study by Cohen-Cline et al. (2015) examines the association between access to green space and mental health among adult twin pairs. This cross-sectional study simultaneously provides outcome data for both exposure to green space and mental illness, including depression, stress, and anxiety, a key benefit of cross-sectional study design (Gordis, 2009). Although cross-sectional studies provide no incidence data due to the lack of information on temporal relationships, the results may be used to establish an association between green space and mental health. Study subjects were identified using same-sex twin pairs from the University of Washington Twin Registry, a community-based sample of twins raised together (Cohen-Cline et al., 2015). The study included 4338 twins that lived, or currently reside, in Washington State, both identical and fraternal (MZ and DZ). The use of both twin types, raised together, reduces genetic and environmental confounding factors found in earlier longitudinal studies (Duncan et al., 2014).
The primary exposure of interest was green space, measured by the Normalized Difference Vegetation Index (NDVI), which uses satellite data to estimate vegetation within a geocoded area (Cohen-Cline et al., 2015). Several mental health outcomes were measured, including depression, stress, and anxiety, each using a specific instrument validated against the Diagnostic and Statistical Manual Fourth Edition (DSM-IV). The tests included the Patient Health Questionnaire, Perceived Stress Scale, and Brief Symptom Inventory (Cohen-Cline et al., 2015).
The hypothesis that greater access to green space would be associated with better mental health outcomes may be confounded by several factors. The study considered household income and physical activity levels as covariate measures, with additional information gathered on the twins through self-reporting (Cohen-Cline et al., 2015). Additional environmental measures included population density and population deprivation, both gathered using census tract data. A multilevel random intercept model was used for data correlation, which in the study included associations between twins within a pair and individuals living within specific tracts, allowing study investigators to compare multiple outcomes.
The study indicated an inverse association between access to green space and mental health, supporting the original hypothesis that greater access to green space is associated with less depression. The study did not support a similar association between green space and stress or anxiety. While unknown confounding factors, both genetic and environmental may have been unaccounted for, results for both identical and fraternal twins, as well as individuals within the specified community, were fairly consistent. Twins who lived in greener areas had higher incomes and physical activity levels, possibly due to self-selection into greener communities that support outdoor activities such as walking and cycling (Cohen-Cline et al., 2015).
As in most studies, there is potential for both selection and information biases. Selecting the subjects from a twin registry reduced many of the potential selection issues, improving the study's internal validity (Gordis, 2009). Information bias may occur at several points in the study, as the instruments used are primarily self-reporting questionnaires, with opportunities for intentional and unintentional misinformation or wish bias (Gordis, 2009). In fact, the primary strength of the study is the use of a large sample of twins raised together, as it controls for unmeasured genetic and environmental confounding factors (Duncan et al., 2014). The cross-sectional design study contributed to the base of knowledge regarding the benefits of green space in community health planning, but it does not allow for causal inference, as reverse causation is not addressed. Perhaps the greatest weakness of the study is the use of 90% white subjects, reducing the generalizability of the results (Cohen-Cline et al., 2015). Further longitudinal follow up studies using twin subjects are necessary to determine potential causal inferences, with additional meta analysis of existing study data (Astell-Burt et al., 2014). Additionally, more data on the types of green space and the effects of each on the overall mental health of a community of interest would provide health officials with a more complete strategy for reducing depression, stress, and anxiety through greater urban planning (Fan et al., 2011).
Alcock, I., White, M. P., Wheeler, B. W., Fleming, L. E., & Depledge, M. H. (2014). Longitudinal effects on mental health of moving to greener and less green urban areas. Environmental science & technology, 48(2), 1247-1255.
Astell-Burt, T., Mitchell, R., & Hartig, T. (2014). The association between green space and mental health varies across the lifecourse. A longitudinal study. Journal of epidemiology and community health, 68(6), 578-583.
Cohen-Cline, H., Turkheimer, E., & Duncan, G. E. (2015). Access to green space, physical activity and mental health: a twin study. Journal of epidemiology and community health, 69(6), 523-529.
Duncan, G. E., Mills, B., Strachan, E., Hurvitz, P., Huang, R., Moudon, A. V., & Turkheimer, E. (2014). Stepping towards causation in studies of neighborhood and environmental effects: How twin research can overcome problems of selection and reverse causation. Health & place, 27, 106-111.
Fan, Y., Das, K. V., & Chen, Q. (2011). Neighborhood green, social support, physical activity, and stress: Assessing the cumulative impact. Health & place, 17(6), 1202-1211.
Gordis, L. (2009). Epidemiology. (5th ed.). Philadelphia, PA: Saunders Elsevier.
Lee, A. C. K., & Maheswaran, R. (2011). The health benefits of urban green spaces: a review of the evidence. Journal of Public Health, 33(2), 212-222.