Unique to mammals, thelimbiccorteximpelscreaturestonurturetheir offspring by delivering feelingsoftendernessandwarmth to the parent whenchildren are nearby.
It is surely no coincidence that much of the activity Dr Okubo found is in that characteristically human part of the brain, the enlarged cerebral cortex, rather than in the limbic system.
One of serotonin's jobs is to show growing nerve cells how they should connect from the frontal cortex, where reasoning takes place, to the limbic system, the seat of many emotional responses.