Love is one of the strongest and most complex emotions. The stages of love fall roughly into three categories - lust, love (romance) and long-term commitment. These different types of love all have different evolutionary reasons and a different effect on the body.

The first stage of a relationship, or the first sign of attraction, is lust. This is mostly driven by a surge of the hormones oestrogen and testosterone . Romantic love, which generally occurs when the relationship has been properly established, is more a result of changes in dopamine and seroton, which are the pleasure related neurotransmitters. A person who is in love will have higher than normal levels of dopamine, yet surprisingly serotonin levels fall (serotonin is often considered the 'happy' chemical in the brain). Atypically low levels of serotonin during the passionate stage of love mimics the brain activity seen in people suffering from obsessive compulsive disorder. Which makes it understandable why someone who is very much in love becomes so infatuated with their partner that they find it hard to concentrate on anything else.

Over time, levels of dopamine and serotonin in the brain will return to more normal levels, yet many couples still stay together after this 'honeymoon period' has worn off. It is believed long term bonding is reliant on the production of hormones such as oxytocin and vasopressin. Scientific studies on animals which are typically monogamous show that suppressing the production of these hormones can break an attachment to partners.

There are good evolutionary reason for these three stages. Initial attraction (lust) is obviously essential to the urge to procreate and continue the species. In humans a relationship is necessary if there is to be a good chance of conception, as it takes the average couple several months to fall pregnant. This would explain the purpose of romantic love. Divorce and family break ups are becoming increasingly common in a society where it is possible to raise healthy children without a two parent household, but in evolutionary terms long term coupling would once have increased the chance of offspring surviving when living conditions were poorer and infant mortality was much higher.