Researchers have determined that romantic love is a complex emotion that can be divided into either passionate or companionate forms. Berscheid and Walster (1978) and Hatfield (1988) found that these two forms can co-exist, either simultaneously or intermittently. Passionate love is an arousal-driven emotion that often gives people extreme feelings of happiness, and can also give people feelings of anguish. Companionate love is a form that creates a steadfast bond between two people, and gives people feelings of peace. Researchers have described the stage of passionate love as "being on cocaine", since during that stage the brain releases the same neurotransmitter, dopamine, as when cocaine is being used. It is also estimated that passionate love (as with limerence) lasts for about twelve to eighteen months.