A study analyzing the relationship between ecological, socio-economic, and psychological parameters in ninety-three states established a positive correlation between the population's happiness level and the density of the cat population in that country.
Furthermore, the analysis revealed an inverse dependence: a high frequency of detecting immunoglobulin M to Toxoplasma gondii in blood serum negatively correlates with the level of happiness. It is worth noting that Toxoplasma gondii is the causative agent of a parasitic disease that can be transmitted through cat feces.
The results of this scientific analysis were officially published in the publication PLoS One.