title: Re-visiting the relationship between neighbourhood environment and BMI: an instrumental variables approach to correcting for residential selection bias.
abstract: BACKGROUND
  A burgeoning literature links attributes of neighbourhoods' built environments to residents' physical activity, food and transportation choices, weight, and/or obesity risk. In cross-sectional studies, non-random residential selection impedes researchers' ability to conclude that neighbourhood environments cause these outcomes.
  
  
  METHODS
  Cross-sectional data for the current study are based on 14,689 non-Hispanic white women living in Salt Lake County, Utah, USA. Instrumental variables techniques are used to adjust for the possibility that neighbourhoods may affect weight but heavier or lighter women may also choose to live in certain neighbourhoods. All analyses control for the average BMI of siblings and thus familial predisposition for overweight/obesity, which is often an omitted variable in past studies.
  
  
  RESULTS
  We find that cross-sectional analyses relating neighbourhood characteristics to BMI understate the strength of the relationship if they do not make statistical adjustments for the decision to live in a walkable neighbourhood. Standard cross-sectional estimation reveals no significant relationship between neighbourhood walkability and BMI. However, the instrumental variables estimates reveal statistically significant effects.
  
  
  CONCLUSIONS
  We find evidence that residential selection leads to an understatement of the causal effects of neighbourhood walkability features on BMI. Although caution should be used in generalizing from research done with one demographic group in a single locale, our findings support the contention that public policies designed to alter neighbourhood walkability may moderately affect the BMI of large numbers of individuals.
authors: Department of Family and Consumer Studies, University of Utah, 225 South 1400 East, Room 220, 84112, Salt Lake City, UT 84112, USA. zick@fcs.utah.edu
authors: Cathleen D
authors: CD
authors: Zick
authors: Heidi
authors: H
authors: Hanson
authors: Jessie X
authors: JX
authors: Fan
authors: Ken R
authors: KR
authors: Smith
authors: Lori
authors: L
authors: Kowaleski-Jones
authors: Barbara B
authors: BB
authors: Brown
authors: Ikuho
authors: I
authors: Yamada
mesh_terms: D000328:Adult; D015982:Bias; D015992:Body Mass Index; D001835:Body Weight; D003430:Cross-Sectional Studies; D004777:Environment; D004779:Environment Design; D005260:Female; D015438:Health Behavior; D006801:Humans; D009765:Obesity / Q000209:etiology*; D011247:Pregnancy; D012111:Residence Characteristics; D014589:Utah; D016138:Walking; D044465:White People; D055815:Young Adult