The report, published Nov. 10 in BMJ, a prestigious British medical journal, was a “meta-analysis,” which involves reconciling data from numerous past studies in order to form a general conclusion.
“It was very fulfilling to put something empirical on paper because this field has been fairly ideological in the past,” said Dr. S V Subramanian, an associate professor in the department of Society, Human Development, and Health at HSPH and one of the authors of the study.
Subramanian said he is hopeful that, after further research, the report’s conclusions can go beyond the realm of science to enter policy arena.
“If [income inequality] is indeed a risk factor, the natural implication should be that we should do something about it,” he said.
The study quantified income inequality using a statistical constant, the “Gini coefficient.” When the coefficient is 0, income is distributed equally across the population. As inequality increases, the coefficient approaches 1.
The report tentatively established 0.3 as a “threshold” coefficient after which income inequality causes deleterious effects on health.
“The important thing in this paper is the verification of the threshold effect, not the estimated numbers,” said Dr. Naoki Kondo, another author of the study.
The numbers presented are inherently estimations because of the differences between the studies analyzed, said Kondo, who is a professor at the University of Yamanashi in Japan.
“Now that we have seen a pattern,” Subramanian said, “we must find out whether the effect is causal.”
He provided two hypotheses that may explain the observed connection.
First, societies with larger income gaps may have policies that foster inequality in health care or education, which could adversely affect health, he said.
The second hypothesis, which Subramanian finds “more intriguing,” is that in unequal societies the “psychosocial stress” of competition may negatively impact cardiovascular health and the immune system.
Sociology Professor Dr. Jason Beckfield wrote in an email that the analysis in BMJ opens the door to further research.
“I think we need more evidence to derive clear implications for policy-makers, but the BMJ article helpfully points a way ahead to better science on this crucial question,” he wrote.