High levels of the trait psychoticism were believed by Eysenck to be linked to increased vulnerability to psychosis such as schizophrenia. He also believed that blood relatives of psychotics would show high levels of psychoticism, suggesting a genetic basis to the trait. Critics of the psychoticism have suggested that the trait is too heterogeneous to be taken as a single trait. For example, in a correlation study by Donald Johnson (reported in 1994 at the APT International Conference) Psychoticism was found to correlate with Big Five traits Conscientiousness and Agreeableness; (which in turn correlated strongly with, respectively, MBTI Judging/Perceiving, and Thinking/Feeling). Thus, Costa and McCrae believe that agreeableness and conscientiousness (both which represent low levels of psychoticism) need to be distinguished in personality models. Eysenck also argued that there might be a correlation between psychoticism and creativity. Psychoticism is believed to be associated with levels of dopamine. Other biological correlates of psychoticism include low conditionability and low levels of monoamine oxidase; beta-hydroxylase, cortisol, norepinephrine in cerebrospinal fluid also appear relevant to psychoticism level.