Therapy animals are everywhere. Proof that they help is not.

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, 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 the critters deserve the credit or something else is at play.