In 1959, researchers such as John Money and Anke Erhardt proposed the prenatal hormone theory. Their research argues that sexual organs bathe the embryo with hormones in the womb, resulting in the birth of an individual with a distinctively male or female brain; this was suggested by some to "predict future behavioral development in a masculine or feminine direction". This theory, however, has been criticized on theoretical and empirical grounds and remains controversial. In 2005, scientific research investigating sex differences in psychology showed that gender expectations and stereotype threat affect behavior, and a person's gender identity can develop as early as three years of age. Money also argued that gender identity is formed during a child's first three years.