United just barred emotional-support pets from flights longer than eight hours. Delta made a similar move last month.

Animals have been peeing, defecating, or mauling passengers onboard flights, and airlines are starting to crack down.

Unlike service dogs, "emotional support animals" are an unregulated group. There's little scientific evidence about what they really do for people.

Emotional support peacocks. Emotional support snakes. Emotional support hamsters.

People have brought all sorts of "support animals" into public places recently, arguing the creatures should be allowed to fly on planes or come into offices because they serve a mental-health purpose.

But the trend has led to a spike in in-flight problems for airlines. Animals have peed, defecated, bit, and in at least one case mauled people on planes. So starting Monday, emotional support pets will no longer be allowed on any United flights longer than eight hours. (Delta adopted a similar rule last month.)

When it comes to the science behind the concept of a support pet, "the research is quite inconsistent on whether the animals really do anything at all," Forensic psychologist Jeffrey Younggren from the University of New Mexico told Business Insider.

By law, a peacock cannot be a service animal. Thomson Reuters

Younggren has spent years studying the trend of patients asking their therapists to sign letters certifying that they need an emotional support animal.

Overall, he said, scientists don't know if such pets do anything "other than make somebody happy."

But despite that lack of evidence, many therapists sign "ESP" letters for their patients, sometimes without even seeing the animals in action.

"How can you say the animal does something if you've never seen them with a patient?" Younggren said.

New rules for United and Delta flights

As such signed letters get more common, some pet owners are using the designation as a way to let their pets fly on planes with them for free, and above the cargo hold.

United says the problem is that many pets simply aren't ready to be cooped up on long flights. The airline is also cracking down on what kind of emotional support animals will be accepted on board: only dogs and cats are allowed now, and no young pups or kittens can come into the cabin.

"We will no longer accept kittens or puppies under four months of age as emotional support animals, in-cabin pets or service animals on any flight, regardless of length," United said in a blog post, explaining the pets often haven't had all their vaccinations yet when they're so young.

But there may be an unspoken reason that some United flyers choose to keep an emotional support pet above the cargo hold: in 2017, of the 24 animals that died in transit on US carriers, 18 perished on United flights, far more dead pets than any other airline.

Meanwhile, Delta said it has seen an explosive 84% increase in incidents involving unruly animals since 2016. Last month, that airline was the first to announce a major crack down, with a similar eight-plus-hour flight ban and four-month-old support pet rule. No exceptions will be granted after February 1, 2019 on Delta, and all tickets purchased after January 3, 2019 on United are subject to the new rules.

Last year, Delta started requiring anyone flying with an emotional-support pet to sign a waiver stating that the animal can behave on a flight. The airline has also initiated other prior restrictions, including requiring proof of vaccination for the animals and only accepting certification letters from a doctor or mental health professional.

What is an emotional support animal?

There's not much regulation about what constitutes an emotional support pet. People can buy their way into a designation pretty easily online for around $70.

Researchers in California looked at more than a decade of records of registered "assistance" dogs and found that from 1999-2012, there was a huge uptick in the number of smaller dogs, older dogs, and dogs used for psychiatric and medical assistance in the state. Those researchers said the study revealed a growing trend of "misunderstanding" and "misuse" of support dogs.

Support pets are not the same as service animals. According to the Americans with Disabilities Act, a service animal must be trained to perform tasks for a person with a disability, be it physical or psychiatric. Disabilities can include being blind or deaf, using a wheelchair, relying on a dog to remind you to take meds, or having a dog around in case of an anxiety attack.

Under federal law, only dogs and miniature horses weighing less than 100 pounds qualify for the "service animal" designation.

These trained animals are on the job and allowed to accompany their humans anywhere that members of the general public can go (including businesses, hospitals, and just about anywhere that's not a sterile operating room).

But the law is clear: "Service animals are working animals, not pets." The ADA even spells out that "dogs whose sole function is to provide comfort or emotional support do not qualify as service animals under the ADA."

The Fair Housing Act, however, is a bit more lenient: It says that US tenants have a right to keep "assistance animals," including emotional support pets, in their homes even if a leaser has a strict no-pets policy.

Therapy dogs are a third category of animal, and they're trained to help calm patients down during therapy sessions, usually in clinical settings.

Animals can help people feel better, but they have to be trained

People who train and certify dogs to work with patients are worried about the growing number of untrained pets flying on planes.

Alice Smith, a client services coordinator at the PAWS dog training center in Florida, told Business Insider that untrained pets are giving real service dogs a bad name.

"There are people who just wanna be able to take their dogs with them everywhere, and they go online and buy a vest," Smith said. She added that if owners don't put in the six months to a year required to train an animal, the dog can end up barking and acting out in public.

However, Smith believes dogs can help for people dealing with anxiety and depression. As a pet owner herself, she said she has benefitted from having dogs around when she's upset.

"My dogs have just known it," she said. "They would come over to me, and get close to me, and as soon as I would pet them, I would calm down."

Smith said there are likely many other people who'd benefit from having a furry, well-behaved friend nearby. She said she even fielded calls from students in Florida who were scared about getting on the bus after the school shooting in Parkland and thought a support dog might help. Other kids call the training center because they're getting bullied and want an emotional support dog to help them get through the day safely. Dogs can also help guide their owners to exits in a panic, or just lean into a person to calm them down in a crowd.

"They can feel that dog's pressure, and know the dog's there," Smith said.

But Younggren pointed out that some people are afraid of dogs or allergic to them. For those individuals, a flight alongside an emotional support pet could be an anxiety- or sneeze-provoking experience.

It boils down to a simple, well-known problem, he said: "People who love dogs think everybody loves dogs."

Update: This post has been updated to reflect new policies on Delta and United. Jeffrey Younggren, a forensic psychology professor at the University of New Mexico, was at the University of Missouri when this story was first published.