Big bloom theory

Why do we give people flowers? To offer condolence to those who are grieving. To celebrate. To seduce. To ask for forgiveness.

Why do we give people flowers? To offer condolence to those who are grieving. To celebrate. To seduce. To ask for forgiveness.

We all know intuitively that there is something psychologically powerful about giving flowers; in fact, few objects have such a universally profound effect. Yet the psychological effect of receiving flowers has been the study of little scientific study, despite the fact that they are part of a multibillion-pound international industry. In the US alone, the flower industry is now worth about $5bn (£2.6bn) a year - which suggests that, at the very least, they service a powerful human need. However, a new study in evolutionary psychology argues that flowers are a species unlike practically any other in their ability to induce positive emotion and other profound changes in our mental state. More intriguing, the study suggests that flowers may have actually evolved to exploit this unique impact on humans.

It's an established idea in evolutionary biology that plants have often evolved to induce responses in a wide variety of species, leading to the dispersal or propagation of the plants. It is known that flowers use animals to assist in their reproduction, but this theory proposes that plant-human co-evolution, or even domestication, is based on the emotional rewards that flowers provide. Jeannette Haviland-Jones of the Department of Psychology at Rutgers State University of New Jersey led a team of psychologists and geneticists in conducting research which suggests that we are part of flowers' reproduction strategy. She argues that flowers have been universally cultivated by humans for at least 5,000 years, giving them a massive evolutionary advantage over other plants. She also suggests that it is more than simple coincidence that flowers have taken on shapes and smells that trigger powerful emotional responses.

In order to test their thesis, the Rutgers team visited 150 women at their homes. Each woman was presented with a variety of gifts chosen to elicit a positive response. The gifts included flowers, or fruit and sweets. The participants didn't know that the study was about the effect of the flowers on their mood. Instead, they were told that it was a study about their daily moods, and they would receive a gift for their participation.

Following the presentation of the gift, those receiving flowers were assessed as displaying a much more positive mood than those who got other gifts, and this effect lasted for several days. Participants who received flowers were also more likely to answer positively to social-support questions (contacting people, talking intimately) after they received the flowers than before. The results suggested that flowers influence our secondary socio-emotional behaviours as well as having a strong effect on our immediate emotional behavioural expression.

In the second study, the psychologists observed participants being handed single flowers, or alternative gifts, in a constrained and stressful social situation - inside a lift. A sign was attached to the gifts or flowers, bearing the explanation: "Free Flowers/Gift! The Society of American Florists Supports a Random Act of Kindness Day! People will be receiving flowers/gifts at random, on the elevator. Pass on the kindness!"

Contrary to cultural expectations, both men and women presented with flowers were more likely to smile, to stand closer and to initiate conversation. Several subjects who were given the alternative gift later learnt that flowers were also being handed out, and returned to the elevator and demanded a flower.

The scientists used elevators for this study precisely because the most typical behaviour for lifts that are sparsely occupied is for strangers to retreat to opposite corners. The measured effect that flowers closed the distance between strangers was remarkable. That this did not occur when other gifts were given indicates that the receipt of a gift in itself did not change the relationship between strangers. Something about the flowers not only induced strong positive mood, but brought significant affiliation between strangers.

The third study involved regularly giving flowers to a sample of people. The researchers found not only a profound elevation of mood, but also reliable improvements in other measures of cognitive function like memory.

In this series of experiments, some participants responded with such unusual emotional displays that the psychologists were totally unprepared for them. The delivery experimenters reported that they received hugs and kisses for the flowers. The psychologists were even invited to return to participants' homes for refreshments.

Various evolutionary theories attempt to explain this unusually powerful psychological effect of flowers. One is that our aesthetic preferences for landscapes and other growing things are related to early hominid survival, when these environmental cues would be related to foraging success. Thus, flowers would be preferred because humans became hard-wired to "emotionally" appreciate "beauty" associated with food gathering. Flowers might evoke a positive response because finding them thriving in a particular location then predicted future food-supplies and possibly a better place for rearing progeny. Yet the problem with this theory is that the showy flowers humans seem to find especially attractive generally do not produce edible fruit. We don't get that excited by a cauliflower, and we certainly don't give cauliflowers on a first date.

Instead, the Rutgers psychologists argue that various sensory elements of flowers combine, even serendipitously, to influence mood, so flowers are "super stimuli," directly affecting emotions through multi-channel sensory interactions, and most of the sensory attractors lead to very rapid and profound changes in our mental states.

For example, we have an evolved preference for patterned symmetries because these can be detected easily as a recognisable signal within a wide variety of visual arrays. Similarly, we may have preferences for certain colours because the various colour channels are important in finding ripe fruit against a green background. The preferences for particular colours and symmetry may be separated from their primary evolutionary use and become rewarding more generally. Plants with preferred colours and symmetry that have no other products would therefore be protected and dispersed. We might also have a preference for specific floral odours. Previous research has established that popular colognes, which often have a floral top-note, will actually reduce depression.

The idea that flowering plants with no known food or other basic survival value to us have co-evolved with humans by exploiting an emotional niche instead is very much like the scenario presented for the evolution of dogs. Flowers may be the plant equivalent of companion animals. If this is true, then there is a very real sense in which, when you next give flowers, they are using you just as much as you are using them.

Raj Persaud FRCPsych is the director of the Centre for Public Engagement at the Institute of Psychiatry, King's College London, and the author of The Motivated Mind (Bantam Press, £12.99)