Scratch one more simple explanation for autism off the list. This time it's the idea that children with autism have low levels of oxytocin, often called the "love hormone" because it can make people more trusting and social.
"Our data blew that out of the water," says Karen Parker, a Stanford researcher involved in the most rigorous study yet of autism and oxytocin levels. The study, published Monday in the Proceedings of the National Academy of Sciences, found that children with autism were no more likely than other kids to have low levels of oxytocin in the blood.
The so-called oxytocin-deficit hypothesis has been appealing because social difficulties are a hallmark of autism spectrum disorders. And there have been hints that the social functioning of people with autism improved with a little extra oxytocin, even a single dose.
Oxytocin is a hormone produced by the hypothalamus that has affects both the body and the brain. It speeds up childbirth and also strengthens the relationship between a mother and her child. And in recent years, it has been shown to enhance feelings of trust and empathy.
So Parker and a team of researchers studied oxytocin levels in nearly 200 children, including those with autism, their siblings, and typical kids. "Our hypothesis going in was thinking the kids with autism would have the lowest oxytocin levels, the siblings would be intermediate and the neurotypical controls would be the highest," Parker says. "That clearly wasn't the case."
Instead, the study found that oxytocin levels affected social functioning in both kids with autism and typical kids. "As your oxytocin levels got higher, your social functioning was more enhanced," Parker says.
The study also found a strong genetic influence on oxytocin levels. A child's likelihood of having high or low levels of the hormone depended on whether their parents had high or low levels.
The findings could explain why some people with autism have responded to experimental treatments with oxytocin while others have not, researchers say. "It could be that if a kid has low oxytocin levels then they might benefit," says Simon Gregory, a genomics researcher at Duke University who was not involved in the study. He is part of another group investigating the use of oxytocin to treat people with autism.
Gregory says it's not surprising that children with autism have widely varying levels of oxytocin. "Autism isn't a disease, it's a spectrum" that can't be linked to any one cause, he told Shots.
The new study is important because it is larger and more rigorous that previous research, which produced conflicting results on oxytocin, Gregory says.
But it's not the final word on oxytocin. It's still possible that levels of the hormone are different in the cerebrospinal fluid around the brain, Parker says. A study to find out is already under way, she says.