Other hormones, oxytocin and vasopressin, kick in later and appear to be crucial for forming long-term partnerships. Couples who have been together for several years show increased brain activity associated with these chemicals, when they look at pictures of their partner. Oxytocin is produced when couples have sex and touch, kiss and massage each other - the hormone makes us more trusting, helps overcome "social fear" and is important for bonding.

Brain scans of people in love show that the old adage "love is blind" really is true. While the dopamine reward areas are excited in love, regions linked with negative emotions and critical social judgement switch off.