Genetic and environmental influences on residential location in the US
Highlights
► Genetic and environmental influences on residential walkability of twins were examined. ► In twins 18–24.9 years, residential walkability was influenced by common environment factors. ► In twins 25+ years, residential walkability was influenced by unique environment factors. ► There was a small genetic effect on residential walkability in both age groups. ► These findings confirm the importance of environmental factors in residential walkability.
Introduction
The role of the physical or built environment in supporting healthy lifestyles has gained increased attention over the last decade. However, causal conclusions about the effects of the built environment on health are tenuous because the allocation of people into residential locations is neither random nor simply based on individual choices. People are born in certain environments with certain genetic predispositions that may be associated with important health outcomes. In other words, residential selection and health may exhibit some familial transmission. Moreover, irrespective of shared environment and genes, not everyone can live in the environment or “neighborhood” they prefer based on their own experiences and expectations. The differential allocation of people to locations due to social forces and other selection processes (i.e., social and economic stratification) is addressed in statistical models by adjusting for individual characteristics such as age, sex, race, income, education, and, more recently, housing and land use preferences (Frank et al., 2007, Handy et al., 2006). Even so, the problems of structural and possible familial confounding still exist (Messer, 2007, Oakes, 2004, Oakes, 2006, Oakes and Church, 2007). Statistical models can adjust for known correlates, but omit important factors that are difficult or impossible to measure or unrecognized.
Twin studies provide one method to examine the host of factors that drive the selection of residential locations because the degree of similarity in the choices made within monozygotic (MZ) and dizygotic (DZ) pairs can be partitioned into genetic, as well as shared and non-shared environmental factors. For example, in one co-twin control study, MZ twins lived spatially closer to one another than DZ twins (Neyer, 2002) suggesting that choice of residential location may be partially heritable. Similarly, at least two classical twin studies have investigated genetic and environmental influences on residential location selection. This design typically requires the use of population-based twin registries that compare a phenotype of interest in MZ and DZ pairs. The Australian Twin Registry found strong effects for both genetic and shared environmental factors on residential location, measured as the distance from major urban centers, with heritability influenced by both the sex and age of the twins (Whitfield et al., 2005). As shared environmental effects decreased with age, additive genetic factors increased. In contrast, the Netherlands Twin Register observed both common and unique environmental factors determined residential location of Dutch twins, suggesting differences in choice of residential location by urbanization level were not genetic in origin (Willemsen et al., 2005). Despite their differences, age and geography appear to influence genetic and environmental factors on selection into residential locations.
Notably, the Australian and Dutch studies defined residential location using a categorical description of the urbanization level. One measure that may have implications for public health is an indicator of environmental walkability (Frank et al., 2005). Highly walkable environments can exist across the spectrum of urbanization. Walkability indices are continuous measures that capture how walkable a location is independent of urbanization, applicable in cities, suburbs, and even rural areas. We, therefore, conducted a classical twin study of the genetic and non-genetic environmental factors influencing residential location using a walkability index in a community-based US twin registry. The aims of our study were to: (1) estimate the contribution of common and unique environmental factors to walkability; (2) determine if the common and unique environmental influences on walkability vary by age; and, (3) determine if common and unique environmental influences on walkability are independent of neighborhood socioeconomic status.
Section snippets
Methods
Participants and setting. The study sample consisted of 1,389 same-sex twin pairs from the University of Washington Twin Registry (UWTR). The UWTR is a community-based sample of adult twin pairs assembled from Washington State Department of Licensing records. Details of the construction and characteristics of the Registry are described elsewhere (Afari et al., 2006). Briefly, in Washington State, all new driver license applicants and identification card applicants are asked if they are a member
Results
Complete physical street addresses were available and geocoded for 1317 of 1389 twin pairs. As shown in Table 1, the two groups differed significantly on all demographic characteristics examined (all p<0.05). The older age group had a higher mean walkability index than their younger counterparts (0.44 versus −0.76, p<0.05). Mean walkability for all twins was −0.36 (range, −8.47–26.68), and the corresponding quartiles of walkability were −6.76 (25th percentile), −1.88 (50th percentile), and 4.00
Discussion
In unadjusted models, our results demonstrate that common environment explains the greatest proportion of phenotypic variation in neighborhood walkability among younger twins, with no additive genetic and a small unique environmental influence. In contrast, among older twins, unique environmental influences dominate with smaller and equal contributions of additive genetic and common environmental factors. Our results also suggest that environmental factors are still strongly related to
Acknowledgment
This work was supported by RC2HL103416-02 (Buchwald). The funding source had no role in the study design; in the collection, analysis and interpretation of data; in the writing of the report; and in the decision to submit the paper for publication.
References (19)
- et al.
Stepping towards causation: do built environments or neighborhood and travel preferences explain physical activity, driving, and obesity?
Social Science and Medicine
(2007) - et al.
Linking objectively measured physical activity with objectively measured urban form: findings from SMARTRAQ
American Journal of Preventive Medicine
(2005) The (mis)estimation of neighborhood effects: causal inference for a practicable social epidemiology
Social Science and Medicine
(2004)- et al.
University of Washington Twin Registry: construction and characteristics of a community-based twin registry
Twin Research And Human Genetics
(2006) Factor analysis and AIC
Psychometrika
(1987)- Belden Russonello and Stewart: Research and Communications, 2004. 2004 National Community Preference Survey,...
- et al.
Developer and realtor perspectives on factors that influence development, sale, and perceived demand for activity-friendly communities
Journal of Physical Activity and Health
(2010) - et al.
Determining zygosity in the Vietnam Era Twin Registry: an approach using questionnaires
Clinical Genetics
(1989) - et al.
Self-selection in the relationship between the built environment and walking—Empirical evidence from northern California
Journal of the American Planning Association
(2006)