The next best thing is studies with enough participants who use cannabis on their own, allowing for comparisons with those who do not.
The findings of one such study, published on November 15 in the Proceedings of the National Academy of Sciences USA, highlight symptoms of increased anxiety, hyperactivity and aggression in children whose parents used cannabis during pregnancy.
The results are “very striking, very much a first,” says Daniele Piomelli, a professor and director of the Center for the Study of Cannabis at the University of California, Irvine, who was not involved in the work.
From ages three to six, they also underwent recordings of their heart-rate variability, another indicator of stress response, and evaluations for anxiety, aggression and hyperactivity.
In the placental tissues, gene activity was altered with cannabis exposure during pregnancy: genes related to the inflammatory response showed decreased function.
Anxiety is an example of a potential confounding factor, says Mitch Earleywine, a professor of psychology at the University at Albany, State University of New York, who was not involved in the study.
Hurd agrees that human studies will always involve elements that can muddy the findings.
For parents who used cannabis during pregnancy and find these results potentially unsettling, “the human organism is very resilient,” Piomelli says.
Emily Willingham is a science writer and author of Phallacy: Life Lessons from the Animal Penis .