People lavish affection and money on pets and some even prefer them to humans. John Archer reveals the evolutionary reasons

P D. James's novel The Children of Men takes the apparent decline in sperm counts over the past half century to its logical conclusion. She portrays a future world where no children have been born for 25 years. Among numerous changes in behaviour and social institutions are the use of pets as complete substitutes for children. The novel describes acrimonious custody cases over cats and the baptism of kittens by Church of England clergy.

Even a cursory examination of today's newspapers shows how far many of us in the western world have already travelled down that road, even in the presence of abundant human infants and children. A report in The Guardian three years ago described pet custody cases as a "new and bitter form of legal battle" in American divorce courts. Owners paid up to $10,000 to ensure custody, and a Florida judge went as far as to examine the dog's new home to make sure that the garden had sufficient shade.

Around the same time, an enterprising former vicar provoked Church of England condemnation by offering (at Pounds 30 an hour) baptisms, marriages and funerals for pets. By offering funerals, he was building on an already widespread market represented by commercial pet cemeteries, dating to pre-Victorian times in the United Kingdom.

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Custody battles and religious services for pets may be extreme examples, but they represent the tip of the iceberg in terms of the enormous amount of affection, time and money people lavish on their pets. More mundane examples include paying for grooming and health care, buying toys and presents, celebrating pets' birthdays, offering rewards when they are lost, and buying them Christmas and Valentine cards. Pet ownership is pervasive in this and other western nations.

People's feelings about their pets are similar to those they have for other humans in close relationships. The pet fulfils the need to nurture and to love. It provides company and a sense of security. People search for pets when they go missing and grieve for them when they die. Systematic research has demonstrated all these features, which conform to the notion of attachment developed in the work of John Bowlby and others.

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Attachment refers to an emotional bond with another individual. When we look for comparable examples in the animal kingdom, we find them restricted to relationships with obvious biological significance, for example between close kin (particularly offspring and parents), or involving shared parenting, or mutual benefits. There are sound Darwinian reasons why attachments should be confined to these sorts of relationships. On the face of it, pets would seem to fall outside these categories. But there are plenty of examples of animals forming prolonged and intimate relationships with members of a different species. These fall into one of three groupings: mutuality, where there is mutual benefit; commensalism, where one benefits but at no cost to the other; or social parasitism, where one benefits at the other's disadvantage.

Some authorities argue that pets provide genuine fitness benefits to their owners and that these exceed the cost of keeping them. But research indicates only minor benefits for health and well-being. To establish an evolutionary or fitness benefit for pet ownership, we would have to show that owners showed a reproductive advantage over nonowners.

In contrast, the substantial increase in reproductive success shown by domestic dogs and cats over their ancestral forms is clear. But this has occurred at some cost to human owners. The money, time and affection lavished on pets could have been channelled into rearing human offspring or helping relatives. This is likely to be important in marginal cases where resources are scarce.

Pets seem to fall into the category of social parasites. In evolutionary (but not conscious) terms they manipulate their human owners. They are the cuckoos in our nests, although a more useful parallel is with the social parasite Atemeles pubicolis, a small beetle that lives in ant colonies. This beetle has, in the words of E. O. Wilson, "broken the code of the social insects". The social behaviour of the ant hosts is controlled by a limited number of signals involving touch and smell. By mimicking these, the parasitic beetle becomes accepted into the ant colony and is cared for.

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In a similar but less extreme way the human pet has broken the code of our parenting reactions. Konrad Lorenz suggested in 1943 that humans respond in a parental way to a set of facial and bodily features that are found in the young of most birds and mammals. These include a large forehead, big eyes and chubby cheeks. The ancient origin and generality of this reaction has meant that we respond to the same features when they are present in the young of other species. We coo and go "ah" to them. An animal that possesses these features is stimulating one of strongest instinctive reactions of the human species. In keeping with this analysis, those species of dogs most favoured as family pets (as opposed to guard dogs) show both bodily and behavioural features characteristic of the juvenile stage of the ancestral wolf.

But possessing baby features merely starts the interaction between potential owner and pet. Whether the human will look after an appealing-looking animal depends on several other features. Historically, dogs and cats have come to be the most popular pets because of their trainability and availability: they are relatively safe and can be accommodated to our way of life.

The ways in which people interact with their pets are modifications of their interactions with other humans. Because dogs and cats have similar emotions and moods to those of humans, we can empathise with them. We can even share object play with them. They show obvious signs of affection and attachment to their owners, which again is appealing and helps to promote bonding to them.

Mutually satisfying interactions over time are the basis for any form of close relationships between humans, and a similar process would seem to underlie attachment to pets. But surely the pet's limited intellect and lack of language will make the process different in this case? Pet owners overcome this obstacle by behaving as if the pet understands and talks to them. One US survey found that most owners talked to their pets and believed that they were sensitive to the owners' feelings.

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Some owners carry the process of bonding with their pets to the extent of preferring them to people. A US survey of veterinary practitioners found that some owners said they would rather lose their spouses than their pets. What they seem to value in their pets, particularly dogs, is their apparently nonjudgemental nature, in contrast to human relationships that are fraught with difficulties.

To sum up, the evolutionary reason why we love our pets is that they take advantage of certain innate human reactions that have evolved to enable us to form biologically important relationships with other humans, particularly with our children. Pets have flourished by so doing, in terms of increasing their numbers. Human owners, once emotionally involved with their pets, will continue to care for them even if they are a drain on resources that would be better directed to human kin.

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John Archer is professor of psychology at the University of Central Lancashire. This article is based on a presentation at the 1997 British Psychological Society annual conference at Herriot-Watt University, on April 5. A fuller version of these ideas is included in a paper to be published in the journal Human Behavior & Evolution.