New data and analyses suggests new explanations for the persistence of alleles that increase the risk of schizophrenia. A 2019 article by Liu, et al. suggested that rapid selection for cognitive and emotional traits during human evolution may have increased the risk of schizophrenia. The hypothesis gained support from their data showing that ancestral alleles increased risk and derived alleles reduced risk. A 2023 article by González-Peñas, et al. confirms and extends the theory with data showing the effect is only for alleles subjected to recent natural selection. See references and crucial illustrations below.

Liu, C., Everall, I., Pantelis, C., & Bousman, C. (2019). Interrogating the Evolutionary Paradox of Schizophrenia: A Novel Framework and Evidence Supporting Recent Negative Selection of Schizophrenia Risk Alleles. Frontiers in Genetics, 10, 389. https://doi.org/10.3389/fgene.2019.00389

González-Peñas, J., de Hoyos, L., Díaz-Caneja, C. M., Andreu-Bernabeu, Á., Stella, C., Gurriarán, X., Fañanás, L., Bobes, J., González-Pinto, A., Crespo-Facorro, B., Martorell, L., Vilella, E., Muntané, G., Molto, M. D., Gonzalez-Piqueras, J. C., Parellada, M., Arango, C., & Costas, J. (2023). Recent natural selection conferred protection against schizophrenia by non-antagonistic pleiotropy. Scientific Reports, 13(1), Article 1. https://doi.org/10.1038/s41598-023-42578-0

From table 4 in the article