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Decomposing ethnic differences in body mass index and obesity rates among New Zealand pre-schoolers



To determine the extent to which ethnic differences in BMI Z-scores and obesity rates could be explained by the differential distribution of demographic (e.g. age), familial (e.g. family income), area (e.g. area deprivation), parental (e.g. immigration status), and birth (e.g. gestational age) characteristics across ethnic groups.


We used data on 4-year-old children born in New Zealand who attended the B4 School Check between the fiscal years of 2010/2011 to 2015/2016, who were resident in the country when the 2013 census was completed (n = 253,260). We implemented an Oaxaca–Blinder decomposition to explain differences in BMI Z-score and obesity between Māori (n = 63,061) and European (n = 139,546) children, and Pacific (n = 21,527) and European children.


Overall, 15.2% of the children were obese and mean BMI Z-score was 0.66 (SD = 1.04). The Oaxaca–Blinder decomposition demonstrated that the difference in obesity rates between Māori and European children would halve if Māori children experienced the same familial and area level conditions as Europeans. If Pacific children had the same characteristics as European children, differences in obesity rates would reduce by approximately one third, but differences in mean BMI Z-scores would only reduce by 16.1%.


The differential distribution of familial, parental, area, and birth characteristics across ethnic groups explain a substantial percentage of the ethnic differences in obesity, especially for Māori compared to European children. However, marked disparities remain.

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We wish to thank Statistics New Zealand for access to these data, and to the Data Lab staff for their speedy and thorough checking of our results. We also note that Dr. Glover was employed at Massey University at the time of the study. This work was conducted for A Better Start National Science Challenge, which is supported by the Ministry of Business, Innovation, and Employment. The funding agency had no role in the design and conduct of the study, management, analyses, interpretation of the results or in the preparation, review or approval of the manuscript.

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Correspondence to Nichola Shackleton.

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Supplementary information

Supplementary table 1. The univariate distribution of the covariates in the sample, and the bivariate distribution between for each covariate with BMI z-score and Obesity

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