Whatever your religious perspective, within the Christmas story sits an everyday miracle that's often overlooked: the physical transformation that can happen to a man when he becomes a father.
Thanks to advances in neuroscience, we now have a pretty good idea of the physical changes Joseph will have experienced as Jesus's adoptive father, holding Jesus in his arms for the first time, all those centuries ago.
The scientific evidence
A growing body of evidence shows that men are biologically wired to be parents - and that when they become fathers, adoptive or otherwise, their brains and bodies can undergo dramatic changes to prepare them for their new role.
So we can safely assume that Joseph's testosterone levels, already lowered as a result of his being in a long-term relationship with Mary, will have dropped further as he began to settle into life as Jesus's father.
The levels of other hormones in Joseph's body will also have changed - including increases in oxytocin (the bonding hormone) and dopamine (the reward hormone) - in response to the 'give and take' of his communication with and responses to Jesus's needs.
We don't know how involved Joseph was as a hands-on caregiver, of course, and this will have affected the extent of the physical changes he underwent. Scientists have shown that brain changes occur in fathers as well as mothers, but also that they vary according to how much parents 'take the lead' as primary caregivers.
The extent to which fathers believe in, and feel confident about, their caregiving role are also associated with brain differences.
Take the hypothalamus, a part of the brain responsible for keeping the body balanced (it's involved in regulating temperature, food intake, and stress reponse, for example), and an important site of oxytocin synthesis. This key brain region tends to be larger in fathers with the strongest caregiving beliefs, who are aware of their importance for their children, and feel the most positive and rewarded when interacting with their children.
We even have evidence that changes in men's brains may differ according to the public policy context in which they navigate early fatherhood. A study comparing US and Spanish fathers discovered significant differences in the extent of brain changes they experienced, with the researchers suggesting that the Spanish dads' bigger brain changes may have resulted from their having had access to paid paternity leave, while the US fathers did not.
So the involved fatherhood miracle is both universal and context-dependent - as much now as it was in Joseph's time. If we want more men to experience it (and we should, if we're serious about reducing gender-based, health and educational inequalities) there's a clear solution: enable them to spend more time with their babies.
Paternity and parental leave policy
Back when Jesus was born, Joseph wouldn't have had access to paternity leave, of course: that's a modern invention that only dates back as far as 1993, when it was first introduced in Norway. And depressingly, if we transpose the Christmas sotry to contemporary times, he'd be no better off now.
In modern-day Israel, where Joseph's hometown of Nazareth is now located, fathers have no paternity leave, although they are legally allowed to take six days' leave after the birth, with three of these designated as sick leave and the others as annual leave. In Palestine, where Jesus's birthplace Bethlehem is now located, fathers are entitled to three days' paternity leave.
Joseph was self-employed (as a carpenter) though, so he wouldn't qualify for any of these provisions. And the same would be true if he was based in the UK, where self-employed fathers also have no entitlement to paternity leave, let alone any individual right to parental leave.
Earlier this year, a TUC survey found seven in ten self-employed UK fathers reporting that they took no time off when their baby was bord - meaning they had less opportunity to experience the 'involved father' miracle we know might otherwise be open to them.
What can you do?
That's why we're calling for six weeks' well-paid leave for all fathers in their baby's first year - including a Paternity Allowance, similar to the current Maternity Allowance, for those who are self-employed. If we can reach our goal of 10,000 signatures to our Parliamentary petition by 10 January, it will send a strong sign to the Government that fathers missing out in this way is no longer acceptable.
This Christmas let's pledge to make the 'involved father' miracle available to all fathers - for their own sake, but also for the children and mothers who love and need their active participation in caregiving.