Most epidemiological studies examine how risk factors relate to average difference in outcomes (linear regression) or odds of a binary outcome (logistic regression); they do not explicitly examine whether risk factors are associated differentially across the distribution of the health outcome investigated. This paper documents a phenomenon found repeatedly in the minority of epidemiological studies which do this (via quantile regression): associations between a range of established risk factors and body mass index (BMI) are progressively stronger in the upper ends of the BMI distribution. In this paper, we document this finding and provide illustrative evidence of it in the 1958 British birth cohort study. Associations of low childhood socio-economic position, high maternal weight, low childhood general cognition and adult physical inactivity with higher BMI are larger at the upper end of the BMI distribution, on both absolute and relative scales. For example, effect estimates for socio-economic position and childhood cognition were around three times larger at the 90th compared with 10th quantile, while effect estimates for physical inactivity were increasingly larger from the 50th to 90th quantiles, yet null at lower quantiles. We provide potential explanations for these findings and discuss implications. Risk factors may have larger causal effects among those in worse health, and these effects may not be discovered when health is only examined in average terms. In such scenarios, population-based approaches to intervention may have larger benefits than anticipated when assuming equivalent benefit across the population. Further research is needed to understand why effect estimates differ across the BMI outcome distribution and to investigate whether differential effects exist for other physical and mental health outcomes.

中文翻译：

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人口健康分布的决定因素：检查体重指数的示意图。

大多数流行病学研究都研究了危险因素如何与平均结局差异（线性回归）或二进制结果的几率（逻辑回归）相关；他们没有明确检查在所调查的健康结果分布中风险因素是否有相关性。本文记录了在少数流行病学研究中（通过分位数回归）反复发现的一种现象：一系列确定的危险因素与体重指数（BMI）之间的关联在BMI分布的上限逐渐增强。在本文中，我们记录了这一发现，并在1958年英国出生队列研究中提供了说明性证据。儿童社会经济地位低，孕妇体重高，无论是绝对的还是相对的，在总体BMI分布的上限，儿童的总体认知能力低下和成年人的BMI较高都没有运动。例如，对社会经济地位和儿童认知的影响估计在第90位时是第10个分位数的三倍左右，而对身体不活动的影响估计从第50位到第90个分位数越来越大，而在较低的分位则无效。我们为这些发现提供了可能的解释并讨论了含义。在健康状况较差的人群中，危险因素可能具有更大的因果关系，而仅以平均水平检查健康状况时，可能不会发现这些影响。在这种情况下，以人口为基础的干预方法可能会带来比预期的更大的收益（假设整个人口都享有同等收益）。