Dads – do you feel a bit of a spare part? Do you see yourself in relation to your kids as about as useful as a stick of furniture – one of the lesser used pieces, like the side table in the guest bedroom? Well, the good news, delivered in a BBC documentary just in time for Father's Day, is that those traditional dad activities, such as talking over your children's head with long words and complex sentences, or swinging them around so that their shoulders threaten to come out of their sockets, are biologically useful. Fathers make evolutionary sense – and not just as reconstructed pseudo-mummies, or "new dads".
"There's a lot of talk at the moment about the absence of fathers, and a curiosity about what it is that fathers actually do," says child psychologist Laverne Antrobus, presenter of "The Biology of Dads", a one-off documentary in BBC Four's Fatherhood season. "In some ways, parenting has been merged. Fathers have been invited to be like mothers rather than to be like fathers – this idea of everything being about nurture. So I wondered what it must be like for fathers who think, 'What do I do that's unique and a bit different?'" Antrobus has collated the latest academic research on the subject, including the already well understood role of the kind of rough-and-tumble games preferred by fathers, which teach toddlers the boundaries of aggression and discipline.
Less well known is the part fathers play in language development – conversing with their toddlers, seemingly inappropriately, as if with a fellow adult. "Mums are constantly adapting their vocabulary so a child knows the word, but dads spur them on," says Antrobus. "They not only use longer words, but they encourage more complex uses of language, such as wit and sarcasm."
Never mind that sarcasm is the lowest form of wit, the gist of Antrobus's film is that fathers consistently push boundaries and nurture independence. What men might find less palatable is what has been happening to their hormones in the meantime. For in a very real, biological sense, fathers of newborn children become more like women.
Now, I remember my wife regularly declaring that her hormones "were all over the shop" while she was pregnant, and in the months after she gave birth to our daughter. Little did I suspect that my own hormones were also on the march. Firstly, it seems that the female hormone prolactin, which triggers lactation and the urge to breast-feed, and which lies dormant in men, springs into action in fathers-to-be. Indeed there is a syndrome known as Couvade Syndrome, or "sympathetic pregnancy", whereby the partners of pregnant women report feelings of morning sickness as well as the urge to binge eat unusual foods. Simultaneously, the key male hormone, testosterone, the one strongly associated with aggression, goes into free fall, back to levels not experienced since before puberty.
Dr Nick Neave, an evolutionary psychologist at Northumbria University, reckons this is nature's way of curbing a new father's behaviour. "You don't want some big, butch, hairy, violent male around these infants, because males who are high in testosterone have low levels of frustration tolerance," he says. "You don't want these guys flying off the handle when the baby starts to cry."
Even so, in the case of "shaken baby syndrome" the overwhelming proportion of perpetrators are men. One can only imagine the carnage if levels of testosterone didn't drop off naturally. And how does this square with anecdotal evidence that suggests that the birth of a child sometimes triggers either physical abuse directed at the mother, or the man loping off to have have an affair with another woman? "We didn't go there (in the film) because if you've got somebody who is prone to domestic violence there will be a lot of other things that are happening at about that time. There will be a dynamic in that relationship that is around vulnerability, for instance", says Antrobus. "In an evolutionary sense, what is really important is that you get this drop in testosterone, thank goodness; it's an opportunity to get the best out of dads."
In her BBC film, Antrobus observes an experiment in which a new father is handed a doll wrapped in his daughter's blanket. His child's pheromones, the chemical messengers that can affect someone else's behaviour, soon have the new father protectively cradling the doll and subconsciously comforting it with blanket tucking and pats to the back. The experiment is repeated with a testosterone-packed single male, who might as well be – in Laverne's words – "holding a sack of spuds".
A father's testosterone levels creep back up as the child grows and becomes less vulnerable, at which stage his replenished masculinity, as we have seen, starts to have a beneficial effect on the child's development. This positive influence continues through the teenage years, when a father's less emotional, more unambiguous approach to discipline can often be more effective. And it seems that a father's influence can influence the partner his daughter chooses: a good relationship with her father often means she looks for a mate with similar physical attributes to her male parent. In Antrobus's film a group of students is asked to match for physical likeness photographs of women and their partners with photographs of the women's fathers – an advanced game of snap that they conclude with unerring accuracy.
And then there is a surprising biological effect that a father can have on his daughter. Studies show that girls who grow up with an absent fathers look older and start their periods earlier. It seems that a need for male protection is causing them to reach sexual maturity earlier. All very interesting, but wasn't Antrobus at risk of making single mothers feel inadequate with her film?
"I felt a bit mixed making this because there will be a lot of single mums out there who'll be thinking, 'Gosh, does that mean my kids are not getting x,y or z?' It's really not supposed to say that. Actually I think that the reality for single parents – mums or dads – is that they are tasked with taking both roles, and most of them do that well enough.
"I hope [the film] is taken at face value," she says. "It really felt like there was a need to say, 'Come on dads, you do do something which is uniquely different and which your child is crying out for – so get in there.'
"Despite what we've tried to do over the generations in terms of how mums and dads are perceived, or how men and women are perceived, our children are still very much fixed on 'your mum is the one who looks after you this way, your dad does things slightly different'. And it's important that he does."
'The Biology of Dads' screens on Tuesday 22 June, at 9pm on BBC Four.
What fathers are for
Fathers can spur language development by using less "baby talk" and more adult language, even if the vocabulary seems to fly over the child's head. They can even teach complex verbal skills like wit and sarcasm.
A father's more adventurous play can encourage children to explore beyond their comfort zone, and learn how to take sensible risks. "They are always pushing a child more, when mums are more hesitant," says Laverne Antrobus. "Rough-housing allows the child to exert power but also respect limits."
Fathers can make more effective disciplinarians, especially during the teenage years, adopting less emotive reasoning than mothers. Studies have also found that single fathers find it more important than single mothers to set up routines in the home, from regular meals to bedtimes.
Studies show that girls who grow up with absent fathers start their periods on average 12 months before other girls, and reach sexual maturity earlier, probably in a natural response to the need for male protection. In short, girls without fathers grow up faster.