Stress was quantified by factors such as socioeconomic status or years of education.

However, the researchers did not investigate a potential mechanism for how stress and chemicals might interact to create this effect.

While the researcher saw evidence of stronger adverse developmental effects of prenatal chemical exposures in higher-stressed versus lower-stressed pregnant women, the specific results of these studies varied, making it difficult to define the precise magnitude of the effect.

Each chemical the researchers investigated has been tested only in a small number of animal studies, with variability in the quality of those studies.

"While the evidence on the combined effects of chemicals and stress is new and emerging, it is clearly suggestive of an important question of social justice," co-author Rachel Morello-Frosch, a professor in the Department of

Environmental Science, Policy and Management and the School of Public Health at UC Berkeley, was quoted as saying in a news release. "The bottom line is that poverty-related stress may make people more susceptible to the negative effects of environmental health hazards, and that needs to be a consideration for policymakers and regulators."