The efficacy of therapy animals — a mental health trend seen everywhere from airplanes to college campuses — isn't totally backed up by scientific research, per The Washington Post.

The problem: The research around the topic is relatively scarce and prone to cherry-picking, thanks to small sample sizes or poor, if any, control groups. It's particularly difficult to isolate if a person's lower anxiety levels are due to proximity to an animal or any number of other reasons associated with caring for it.

An explanation: The biophilia hypothesis states that humans evolved a need to enjoy the company of other creatures. That might be difficult to quantify with scientific research but that may not matter:

Why the science matters: Medical applications. WaPo cites the VA refusing to fund service dogs for veterans with PTSD in 2012, which led the department to embark on a multiyear study to determine the efficacy of therapy dogs.