A therapy-animal trend grips the United States. The San Francisco airport now deploys a pig to calm frazzled travelers. Universities nationwide bring dogs (and a donkey) onto campus to soothe students during finals. Llamas comfort hospital patients, pooches provide succor at disaster sites, and horses are used to treat sex addiction.

And that duck on a plane? It might be an emotional-support animal prescribed by a mental health professional.

The trend, which has accelerated hugely since its initial stirrings a few decades ago, is underpinned by a widespread belief that interaction with animals can reduce distress. Certainly, the groups offering up pets think this, as do some mental health professionals. But the popular embrace of pets as therapists is kindling growing discomfort among researchers in the field, who say it has raced far ahead of scientific evidence.

Earlier this year in the Journal of Applied Developmental Science, an introduction to a series of articles on “animal-assisted intervention” said research into its efficacy “remains in its infancy.”

A recent literature review by Molly Crossman, a Yale University doctoral candidate who recently wrapped up one study involving an 8-year-old dog named Pardner, cited a “murky body of evidence” that sometimes has shown positive short-term effects, and occasionally identified higher rates of distress.

Overall, Crossman wrote, animals seem to be helpful in a “small-to-medium” way, but it’s unclear whether the critters deserve the credit or something else is at play.

“It’s a field that has been sort of carried forward by the convictions of practitioners” who have seen patients’ mental health improve after working with or adopting animals, said James Serpell, director of the Center for the Interaction of Animals and Society at the University of Pennsylvania School of Veterinary Medicine. “That kind of thing has almost driven the field, and the research is playing catch-up. In other words, people are recognizing that anecdote isn’t enough.”

Using animals in mental health settings is nothing new. In the 17th century, a Quaker-run retreat in England encouraged mentally ill patients to interact with animals on its grounds. Sigmund Freud often included one of his dogs in psychoanalysis sessions. Yet the subject did not become a research target until the American child psychologist Boris Levinson began writing in the 1960s about the effect his dog Jingles had on patients.

But the evidence to date is problematic, according to Crossman’s review and others before it. Most studies had small sample sizes, she wrote, and an “alarming number” did not control for other possible reasons for a changed stress level. Studies also tend to generalize across animals, she noted: If participants are measurably soothed by one golden retriever, that doesn’t mean another dog will evoke the same response.

Alan Beck, who directs the Center of the Human-Animal Bond at Purdue University, cites one common theory for why animals might be therapeutic. It’s called the biophilia hypothesis, and it argues that humans evolved a built-in need to affiliate with other living beings.

“Throughout history, animals gave us some comfort. So if it works for you and me in a relatively normal environment, maybe it has a special role for someone who has a depression and stress disorder – that just makes sense,” he said.