When politicians want to ingratiate themselves with the electorate they are pictured kissing a baby; not a small child or a little old lady, but an infant. The association with a tiny, weeny, cooing, gurgling little babbakinsy-poos has a special power.

Tony Blair has pulled off a spectacular variation of this political stunt. Not only can he now show off his baby son, he can do so in the run- up to an election. This is particularly desirable for a prime minister who, despite his popularity, has never really been loved or lovable.

But what is most interesting of all is that no one - not even the most hardened of cynical political pundits - seems to doubt that the arrival of Blair Minimus will be a huge electoral asset, adding poll percentage points. No other life event would have the same effect on a politician's standing; not getting married, not being cured of a terrible illness, not even being in charge of the nation when winning the World Cup.

The appeal of the little blighters seems to have exploded just lately, extending beyond the traditional domain of women into the lap or on to the shoulder of men, not all of them New. Take Dennis Wise, a man not known for his maternal, caring sharing ways. When he went up to collect the FA Cup on Saturday he stopped off to pick up his tiny infant. Astonishingly, we Chelsea supporters witnessed the implicit message that winning the cup was less important than showering his babba with kisses.

To a significant extent this act was almost certainly copycat babyism. For it had been only a few days earlier that David Beckham celebrated winning the league title in the company of his 14 month old son Brooklyn. The other Manchester United players also brought their offspring onto the pitch.

Nor is this a bit of local madness infecting soccer players. Across the media, celebrities known for their stereotypically masculine ways are showing signs of baby love.

The Gallagher brothers, supposedly wild men of rock, did not turn up for the launch party of their new album because they were babysitting. Said a besotted Liam of his new son Lennon, "I realise I'm blessed." Damon Albarn, his great rival, is talking of giving up work to look after his baby.

Martin Clunes, who played the misogynistic Gary in the BBC's Men Behaving Badly recently added his voice to the chorus of baby-worhippers. "No matter how many preconceptions you have nothing really prepares you for the second when your baby is born."

What is going on? What accounts for the baby's pulling power?

More than 50 years ago the ethologist Konrad Lorenz suggested that infants are equipped with features that automatically set off a rush of emotions in us. Their cuteness is comprised of soft skin and hair, almost adult-sized eyes from birth, big pupils, chubby cheeks, fully formed hands and feet, and small noses. Their heads are big and their limbs are small and springy.

Evolutionists claim our response to these traits is built into us because infants need to arrive irresistible because without care they die. The basic baby construction is said to have profound meaning to us and to elicit tender feeling for this reason.

As with all evolutionary psychological theories, it is a speculation which cannot be proved or disproved. Certainly, it is true that juvenile traits in animals instinctively evoke nurture. Chicks have stripes, lion cubs have spots and tail rings, and baby chimps have white tail tufts, all of which signal their helplessness. Thus, chimps who lose the tail tuft are not safe from attack. In the same way, it is speculated, the big heads, big eyes, chubby cheeks and little noses of human infants trigger a protective instinct in us.

There is evidence that babies who do not look very babyish because they are ill or deformed or ugly receive less attention than normal-looking ones. The inference is that mothers are genetically primed to seek signs of health and not to waste valuable energy on infants who are a bad biological bet.

It has also been suggested that men are responding to neotonous, infantile features when they are attracted to women. Evolutionists believe that men are attracted to the looks of the helpless and dependent. If infant features automatically evoke nurture then the more infantile a woman looks, the more men she will attract.

Certainly, it is true that male desire is triggered by youthful attributes. A survey of 454 traditional societies showed that the average age of brides was between 12 and 15 years. The extreme youthfulness of the look (and increasingly, the age) of models in developed nations suggests this age group retain their desirability.

In developed nations, when people are asked to create separate generic images for a child, an adult, a man and a woman using a special computer programme, they create very similar ones for female and child and for male and adult. As faces age they become more masculine; to look feminine is to look young.

On top of all this, I would hazard that the appeal of a baby is also its psychological attributes, especially its authenticity.

As soon as offspring can talk there is a loss of innocence. They show signs of the cunning, self-interest and other ugly adult attributes that led to Sartre's dictum "hell is other people."

By contrast, an infant is incapable of pretence. Honest emotion, unmediated by thought and language, can be witnessed on its face. In exactly the same way that a pet is unpretending and can be trusted, a baby denotes uncomplicated truthfulness.

Certainly, when a politician plants his kiss on the baby (women politicians do not seem to do the baby gag) he is emphasising his nurturant potential - "I'm a guy who can care for and show love towards the vulnerable.". But every bit as much, he is also saying, "You can trust me like you can trust a baby. I'm a human being (as well as a lying shit)."

But having said all this, there are also strong grounds for doubting that our intense feelings about babies are nearly as much powered by instinct as evolutionists would maintain. The fact is that much as we may love babies, we also hate them and, in evolutionary terms, until very recently there was a strong case for parents not to get too attached to offspring until they have passed the age of five.

The problem with babies in the past was that they were a dangerous drain on emotional and physical resources. At least one quarter of children died before the age of five. Women gave birth to an average of eight children so the high infant mortality rate would have rocked them twice during their childbearing years unless they avoided caring too passionately for their infants.

As the psychohistorian Lloyd De Mause put it, "The history of childhood is a nightmare from which we have only recently begun to awake. The further back in history one goes, the lower the level of childcare, and the more likely children are to be killed, abandoned, beaten, terrorised and sexually abused." In this view, our passionate love for infants is a relatively new development.

I found plenty of support for De Mause's analysis when carrying out a study of childcare in a village in western Ecuador. It was typical in most respects of what life was like for the majority of the world's population before 1900, with high infant mortality and basic subsistence a constant struggle.

Few of the mothers became intensely attached to their infants. Given the infant mortality rate this made sense. If they fell in love with their babies in the way that mothers in technologically developed societies are supposed to do, they would be putting themselves at grave risk of suffering repeated bereavement.

Other cross-cultural research bears this out. Compared with mothers in urban developed nations, those from agrarian, undeveloped nations are more concerned with meeting the physical needs and protecting the health of the infant than with forming intense relationships through play and talk.

It has even been argued that abuse was a natural, healthy adaptation. Jay Belsky, a key thinker in this debate based at King's College in London, writes that "across the animal kingdom, the mistreatment of progeny is so widespread that it would seem to be as much a part of the 'natural' condition as is sensitive, solicitious parental behaviour."

He points out that in humans as well as animals, the interests of offspring and parents frequently compete rather than coincide. When this competition becomes too threatening to the parent's mental health or physical survival, abuse is liable to happen.

Belsky presents extensive evidence that where instability, unpredictability or unavailability of resources exist for parents, they are more likely to maltreat children. Thus, poverty and low income correlate strongly with abuse and neglect.

In evolutionary terms there were two conflicting pressures at work when it comes to babies. From their standpoint it made great sense to evolve as innately attractive, otherwise they would not survive. But for parents they were a risky investment before the age of five, a large drain on resources it was best not to get too attached to.

In fact, genes may play only a minimal part in our feelings about babies, perhaps giving us a general tendency to feel positively towards them but allowing great variation depending on circumstances. Many individuals, for all sorts of reasons, prefer toddlers or children to helpless infants.

Mothers who work full-time are more likely to believe that genes largely determine the personality of a child and that parental care is relatively unimportant. Full-time mothers are more likely to believe that what they do forms the child. It may well also be that the working mothers prefer children who can talk and with whom they can have a more adult relationship than infants.

Indeed, many parents find the massive dependence of infants alarming and actually depressing. Women who are vulnerable to the illness are are at greatest risk of developing it during the first six months of a child's life.

That parents of both sexes can find infants immensely upsetting is the reason why babies under the age of one are four times more likely than other age groups to be murdered. Nor is violence an unusual response - threequarters of infants have been hit by a parent before their first birthday.

It is easily forgotten that until very recently anyone who could afford to would pass the care of their infant over to a nanny and that the example the aristocracy set was of a nonchalant lack of passion.

Not until his fourth birthday did Charles share the day with Prince Philip (he was absent for six of the first eight birthdays). His mother seems to have been able to contain her enthusiasm for the future king as well. Her biographer reports that on returning to Britain after a six-week absence, the separation from her infant son had not caused "any obvious consternation" to her and therefore she did not "find it neccessary to rush back to him." She caught up on her correspondence and other administration at home, went with her mother to watch one of her horses race and only after four days did she finally visit her two year old son.

It is hard to imagine a mother acting like that today and even harder still to find such a negligent father. We can be sure that whatever the reality, Blair will not be presented as having acted like this. Indeed, one of the strongest pieces of evidence that baby love is largely dictated by social trends is today's unprecedented passion of men for their babies.

A study done in 1960 found that the pupils of women and children dilate when shown a picture of a baby but not the pupils of a man. In those days men had no involvement in baby care and most had no special feeling towards them. But if that research were repeated today and the subjects included the likes of David Beckham or Liam Gallagher, the results would be very different.

Part of the explanation for the new male passion is their increasing presence at the birth, following childbirth classes. Subsequently, they are more expected to change nappies and pull their weight and this greater involvement has increased their awareness of the the charms of the infant.

There are even studies which are now being published that show men experience "sympathetic pregnancies" with their partners, up to and including changes in their hormonal balance.

None of this change is remotely to do with genes. As with all the bogus attempts to explain our psychology by evolution, it turns out that your particular family history and the society you are in are far stronger determinants of your psychology. While genes may give us a slight prejudice in favour of babies over, say, the aged, it is the facts that they are far more likely to survive the early years and pose much less of a threat to the survival of parents that has turned them into an electoral dividend.

It is that, not evolution or God, which Alastair Campbell should thank as he lays his plans to exploit young Leo's charms.

Oliver James is a clinical psychologist. His new book They F*** You Up (Your Mum and Dad) - Nature versus Nurture will be published by Bloomsbury next spring.