In fact, the original four types of temperament (choleric, melancholic, phlegmatic and sanguine) suggested by Hippocrates and Galen resemble mild forms of types of psychiatric disorders described in modern classifications. Moreover, Hippocrates-Galen hypothesis of chemical imbalances as factors of consistent individual differences has also been validated by research in neurochemistry and psychopharmacology, though modern studies attribute this to different compounds. Many studies have examined the relationships between temperament traits (such as impulsivity, sensation seeking, neuroticism, endurance, plasticity, sociability or extraversion) and various neurotransmitter and hormonal systems, (i.e. the very same systems implicated in mental disorders)