The microflora of the gut have more metabolic activity than any organ system in the human body. They live in a complex ecosystem that consists of trillions of microbes with which we live in a critical symbiosis. Studies in mice show that when they are treated with oral antibiotics their behavior can change from outgoing and curious to passive and withdrawn. As the microflora returns to normal so does the original behavior. Studies on mice with a sterile GI tract show that when microbes from an outgoing and curious mouse is transplanted into their gut, they become outgoing and curious. So, the relationship between the microbes of the GI tract and behavior is important. We also know that there is more neurotransmitter receptor sites for serotonin in the gut than in the brain.