It's been blamed for increasing the risk of severe depression, suicide, schizophrenia, personality changes, and poor impulse control, but Toxoplasma gondii - the cat-borne parasite that's infected up to 50 percent of the world's population - doesn't have even close to the amount of influence on our brains as the Internet would have you believe. 

That's the conclusion researchers in the US have made after investigating the links between certain personality traits, cognitive abilities, and mental disorders to T. gondii infection, also known as Toxoplasmosis.

"Our results suggest that a positive test for T. gondii antibodies does not result in increased susceptibility to neuropsychiatric disorders, poor impulse control or impaired neurocognitive ability," the team from Duke University conclude in the study, published in PLoS ONE. "This is, to our knowledge, the most comprehensive assessment of the possible link between T. gondii infection and a variety of impairments in a single cohort."

Past research has found that when mice are infected by T. gondii, they can show signs of an innate loss of fear of cat urine, and impaired working memory, which scientists hypothesise as being a mechanism that helps the parasite complete its life cycle. This has led researchers to investigate if something similar is occurring in human beings. 

Back in 2012, The Atlantic ran a profile piece called "How your cat is making you crazy," that outlined the research of Jaroslav Flegr, an evolutionary biologist at Charles University in the Czech Republic, who told journalist Kathleen McAuliffe that, "Toxoplasma might even kill as many people as malaria, or at least a million people a year."

"[The] 'latent' parasite may be quietly tweaking the connections between our neurons, changing our response to frightening situations, our trust in others, how outgoing we are, and even our preference for certain scents," McAuliffe summarised his hypothesis. "And that's not all. [Flegr] also believes that the organism contributes to car crashes, suicides, and mental disorders such as schizophrenia."

Over the past few years, a number of studies have agreed with Flegr's assertions, with schizophrenia being the most heavily researched correlation. But within that pool of correlating results, potential biological causes have been lacking.

Now researchers from Duke University have published the results of an investigation into what happens when you take a sample of humans and simply try to find differences in personality traits between those who are infected, and those who aren't.

They analysed blood samples taken from 837 New Zealanders at the age of 38, as part of the Dunedin Longitudinal Study, which has been following the participants' physical and mental health since birth. Of these, 28 percent tested positive to Toxoplasma.

The team looked at four hypotheses, and came up with four conclusions:

1. Is T. gondii infection status related to neuropsychiatric conditions (schizophrenia and major depression)?

T. gondii infection was not significantly associated to either of these conditions.

2. Is T. gondii infection related to poor impulse control as reflected in four phenotypes: non-suicidal self-injury, suicide attempt, criminal convictions, and traffic-related offences and accidents?

While they found no association between non-suicidal self-injury, criminal convictions, and traffic-related offences and accidents, suicide attempts were more common in T. gondii-positive people - but only very slightly. 

3. Is T. gondii infection status related to personality differences?

"The personality profiles of individuals who tested positive for T. gondii antibodies were indistinguishable from the personality profiles of individuals who tested negative," they found.

4. Is T. gondii infection related to poorer neurocognitive performance?

T. gondii wasn't correlated with IQ or other measures of cognitive performance, except on one memory test, and again, only very slightly. "Associations between T. gondii infection status and all other tests of neurocognitive functions were not significant," the researchers report.

So why couldn't they find the connections other studies have? The team suggests that the difference between this study and other research is that while psychological impairments have been tested in isolation in previous studies - which runs the risk of selecting for a certain correlations and skewing the results - here they were tested altogether.

"This is, to our knowledge, the most comprehensive assessment of the possible link between T. gondii infection and a variety of impairments in a single cohort," they write. "Previous positive associations have been reported across different studies, often in selected or clinical samples; for example, one study will examine the link to violence, another the link to schizophrenia, and yet another the link to self-injury, and so forth."

While a sample size of 837 isn't exactly large, the Neuroskeptic blog over at Discover cites another study with a sample size of 7,440, and the results were also "mostly negative". 

So can we conclude that T. gondii is perfectly safe for humans? Nope, because in pregnant women and people with impaired immune systems, it can cause very serious illness and even death. And based on the evidence available, it also wouldn't be wise to assert that the parasite is doing nothing at all to us psychologically, even looking at the conclusions of this latest study.

What we can say is that because so many of us have cats at home, and the fact that half of us could be harbouring this parasite, it's been very easy to freak ourselves out about the possible effects, and the results of this study call for a more measured response to something that we're really not even close to understanding.

"It has been observed that the 'hotter' the topic, and as more studies are reported and accumulate, replication becomes more difficult," the study concludes. "If we accept that the findings reported in the present article represent scenario two, then views of the link between T. gondii and aberrant behaviour may need to be tempered accordingly."