Researchers Javier Borráz-León and Dario Maestripieri from the University of Chicago found a connection between the presence of domestic cats and the reported happiness levels of countries. Their findings, published on September 30, 2026, in the journal PLOS One, examined 93 countries chosen from the 149 listed in the global happiness ranking. The team compared happiness scores—based on Gallup surveys from 2022 to 2024—with the number of domestic cats per 10,000 people. They also considered factors like GDP per capita, health spending, and income. Additionally, the researchers looked at the seroprevalence of Toxoplasma gondii, a parasite that can infect humans, particularly in pregnant women. The study found a strong correlation between the density of cats and national happiness, with a value of 0.62. In contrast, the link between happiness and recent infections of Toxoplasma gondii was negative, at -0.54. Together, these four variables—cat density, recent infection, GDP per capita, and health spending—explained 47% of the differences in happiness levels between countries. The parasite's impact on happiness was comparable to the influence of health spending. Toxoplasma gondii is transmitted through cat feces and undercooked meat. Nearly one-third of adults may carry the parasite without knowing it. In some individuals, a past infection may be linked to mental health conditions such as depression, anxiety, or schizophrenia. However, IgM antibodies indicate a recent infection, which can cause inflammation and potentially affect mood. The researchers stressed that their study does not imply that adopting a cat will make someone happier, nor does it allow individuals to determine their infection risk based on national data. The happiness rankings already include factors like income and life expectancy, and the observed correlations at the country level do not prove individual-level effects. Furthermore, the study found that in wealthier countries, where cats are more likely to live indoors and receive veterinary care, exposure to the parasite is lower compared to countries where cats live outdoors and may contaminate the soil. The study's reliance on data from pregnant women introduces a potential bias, as this group was the only one surveyed across all countries. The researchers also noted that their analysis included pet cats, stray cats, and wild cats, which may complicate the results. Factors like urbanization, climate, and sanitation were not included in the model. The survival of Toxoplasma gondii eggs in soil depends on humidity, but the dataset remains freely available for further research. To confirm the findings, future studies would need to track cat owners over time.