“Is Toxoplasma Gondii Infection Related to Brain and Behavior Impairments in Humans? Evidence from a Population-Representative Birth Cohort,” published in PLoS ONE, 2016. Complete article available (open access) online here.
This study  explores the possible connection between infection with the Toxoplasma gondii parasite and a range of cognitive and behavioral impairments. T. gondii is a common parasite that can be transmitted to humans and other animals when cats (both domestic and wild felids) shed infectious egg-like oocysts in their feces .
Results revealed "little evidence that T. gondii was related to increased risk of psychiatric disorder, poor impulse control, personality aberrations or neurocognitive impairment."
Researchers used data from the large-scale Dunedin (New Zealand) Multidisciplinary Health and Development Study, “a longitudinal investigation of health and behavior in a population-representative birth cohort.” Their results revealed “little evidence that T. gondii was related to increased risk of psychiatric disorder, poor impulse control, personality aberrations or neurocognitive impairment” .
Among the cognitive and behavioral impairments evaluated, these researchers reported that T. gondii-infected individuals were “marginally” more likely to attempt suicide and “also performed worse on one out of 14 measures of neuropsychological function” . No relationship was found between infection and the other outcomes evaluated.
Unlike some studies on the topic, this research employed a reasonably large sample (837 participants) from a larger longitudinal birth-cohort study.
“Assessments were carried out at ages 3, 5, 7, 9, 11, 13, 15, 18, 21, 26, 32, and most recently, 38 years, when [researchers] assessed 95% of the 1,007 study members who were still alive” . Such research design offers a number of advantages, including the ability to reduce the likelihood of false associations otherwise made due to differences in age-related exposures.
Like the authors of a 2017 study published in Psychological Medicine , researchers note here that their results differ from those of some previous studies. They observe that earlier investigations might have been flawed, in part, because of today’s “frustrating scientific search for biological causes with large effects in common mental disorders and processes.”
“Views of the link between T. gondii and aberrant behavior may need to be tempered accordingly.”
Given the shortcomings common to many earlier studies (e.g., small, non-random samples, lack of control for confounding variables), the authors of this study suggest that “views of the link between T. gondii and aberrant behavior may need to be tempered accordingly."
Sugden, K.; Moffitt, T. E.; Pinto, L.; Poulton, R.; Williams, B. S.; Caspi, A. Is Toxoplasma Gondii Infection Related to Brain and Behavior Impairments in Humans? Evidence from a Population-Representative Birth Cohort. PLoS ONE 2016, 11 (2), e0148435.
CDC. Toxoplasmosis: Epidemiology & Risk Factors; Centers for Disease Congtrol and Prevention, 2018.
Solmi, F.; Hayes, J. F.; Lewis, G.; Kirkbride, J. B. Curiosity Killed the Cat: No Evidence of an Association between Cat Ownership and Psychotic Symptoms at Ages 13 and 18 Years in a UK General Population Cohort. Psychological Medicine 2017, 1–9.