Schizophrenia is one of those disorders we still don't know the cause of, but it's likely to involve a complex mix of genes and environmental factors a person is exposed to in their childhood. According to new research, one such factor might have something to do with having pets.

A new study has found a relationship between dog ownership in our early years and a reduced likelihood of developing the neurological condition. Just how the two are linked isn't clear, but in context of research implicating a dysfunctional immune system, this tantalising conclusion demands a closer look.

"Serious psychiatric disorders have been associated with alterations in the immune system linked to environmental exposures in early life, says pediatrist Robert Yolken from Johns Hopkins Children's Centre, Maryland.

"Since household pets are often among the first things with which children have close contact, it was logical for us to explore the possibilities of a connection between the two." 

Having a pet roam around the family home when a child is still crawling around through the dust and dander has long been considered a good way to build a healthy immunity.

Past studies suggest keeping a cat might help to reduce the chances of developing asthma, for example. Microbes common in pets might also help with lowering the risk of obesity later in life.

That said, it's not always a clear win for pets. It can be a challenge for researchers to effectively distinguish the influence of an animal from factors that often come with owning a pet, making such results less convincing in the long run.

Earlier this year a massive study found a parasite commonly found in cats could increase the risk of schizophrenia; the question of how pets affect neurological development is far from a simple one to answer.

To dig further into the problem, Yolken recruited more than 1,300 adult volunteers through inpatient, day hospital, and rehabilitation programs at Sheppard Pratt Health System in Baltimore.

The sample group was then divided into three categories; 381 individuals with a bipolar diagnosis, 396 with a diagnosis of schizophrenia, and 594 with neither.

Schizophrenia and bipolar are both serious neurological conditions affecting millions worldwide, and may even share certain underlying pathways and genes. There's also a growing body of research suggesting a dysfunctional immune system might play a role in their development.

For all their similarities, the two conditions manifest in very different ways; schizophrenia reflecting psychoses that involve delusions and hallucinations, and bipolar periodically presenting as symptoms of mania and severe depression.

The researchers asked their volunteers about their history of pet ownership, dating from their early teen years all the way back to birth.

Nearly 56 percent of those with schizophrenia reported having a dog at some point in their childhood, while 35 percent said they'd had a cat. For people with bipolar, those numbers were 65 percent and 41 percent, and 62 and 35 percent for those with neither.

To see if the figures hid a deeper meaning, the team used statistical tools to determine something called a hazard ratio for each pet during four different age brackets.

Statistically speaking, there were some small trends in owning a cat in the lead-up to adolescence associated with an increased risk of schizophrenia, and a slightly higher chance of having developed bipolar if you had a cat between birth and three years of age.

But the biggest difference by far was a 24 percent lower chance of developing schizophrenia if you had a dog early in life.

"The largest apparent protective effect was found for children who had a household pet dog at birth or were first exposed after birth but before age three," says Yolken.

Yolken tentatively suggests that if we were to presume the statistics reflect a direct, causative link, 840,000 cases of schizophrenia could be prevented by having a pet dog around.

Until further research helps us better understand if such a relationship exists, it's important to keep in mind this claim is rather speculative. Even with more research, we might identify better ways to produce such benefits without needing to necessarily welcome a pet into the home.

"There are several plausible explanations for this possible 'protective' effect from contact with dogs – perhaps something in the canine microbiome that gets passed to humans and bolsters the immune system against or subdues a genetic predisposition to schizophrenia," says Yolken.

Studies like this should never be taken in isolation, especially when making a decision on whether to take on responsibility for a pet. Statistics based on self-reported assessments can be notoriously fickle, so we should always take them in context of a wider field of study.

We're going to need studies like these if we're going help those with schizophrenia overcome challenges posed by their condition.

"A better understanding of the mechanisms underlying the associations between pet exposure and psychiatric disorders would allow us to develop appropriate prevention and treatment strategies," Yolken says.

This research was published in PLOS One.