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Washington: A therapy-animal trend has gripped the US. Universities nationwide bring dogs and other animals on to campus to soothe students during finals. Llamas comfort hospital patients, pooches provide succour at disaster sites and horses are used to treat addictions.

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 — whether it happens over brief caresses at the airport or in long-term relationships at home. Certainly, the groups offering up pets think this, as do some mental health professionals.

But the popular embrace of pets as furry therapists is kindling growing discomfort among some 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 eight-year-old dog named Pardner, cited a “murky body of evidence” that sometimes has shown positive short-term effects, often found no effect 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 they 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 Centre 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 recognising 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 positive 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, such as interaction with the animal’s human handler. Studies also tend to generalise across animals, she noted: If participants are measurably soothed by one golden retriever, that doesn’t mean another species will evoke the same response.

Even so, media headlines are often about the happiness bounce. Hal Herzog, a Western Carolina University psychologist who has long studied human-animal interactions, recalls a 2015 study on the health benefits for children of having a pet dog. “Here’s a reason to get a puppy,” NBC announced. “Kids with pets have less anxiety.”

That’s actually not what the study concluded. The authors did find that children with dogs had lower anxiety based on screening scores than children without dogs. Still, they cautioned that “this study does not answer whether pet dogs have direct effects on children’s mental health or whether other factors associated with acquisition of a pet dog benefit their mental health.”

It was a classic case of conflating correlation and causation, which Herzog says is common. Cherry-picked positive results also are a problem, as he says happens in promotional materials from the Human-Animal Bond Research Initiative (HABRI). The pet-industry backed organisation funds research on the topic.

“The number of papers I see that start out, ‘It is now well-established that there are health benefits from owning pets’ — that drives me crazy,” Herzog said. “Yes, there’s literature that supports that. But there’s also literature that doesn’t find that.”

To many animal lovers and pet owners, the back-and-forth might sound horribly wonky. There’s something intuitive about the good feelings animals give us. Why over-analyse it?

Alan Beck does not disagree. Beck, who directs the Centre 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.

Focusing too much on scientific support sometimes feels like a form of “physics envy,” Beck added, “where you try to quantify everything without appreciating it.”