Do you care about the planet? Don't we all? Well, one lobby supporting environmental causes is the One Planet, One Child glossy billboard campaign now flourishing across Canada and parts of the US and aiming for a more global impact. One Planet, One Child urges couples - the pictures show a conventional male-female couple - to limit their family to one child.
Climate change, traffic congestion, pollution, the degradation of the earth and the decline in mammals, birds and fish species - even wars - are all down to this culprit: overpopulation.
"The best gift you can give your first child is not to have another," is one message. "Conservation begins at contraception," says another, showing a snuggling couple. "Congestion begins at conception," says another, showing a hellish traffic jam - messages all sponsored by the World Population Balance campaign.
Over-population crusaders say that there are just too many of us on the planet at 7.8 billion. Opponents counter that human beings always find new and enterprising ways of using our resources. People aren't the problem - it's wasteful lifestyles that cause pollution, destruction of the environment and animal life.
And in Europe - and many other countries - population is now below replacement levels anyway. Population replacement is reckoned to be 2.1 children per woman; the EU nations are all below that. France and Sweden are highest on 1.88 and 1.76 respectively.
What interests me is not so much the big arguments about population, but the psychology, and the social impact that one-child families might have.
Since the dawn of time, most individuals have had brothers and sisters as part of the family pattern and thus aunts, uncles and cousins.
A friend of mine, who is an only child, married another only child. She was always concerned that her own children would have no cousins.
Significantly, because she was an only one, she was keen to have several, so there would be siblings (she had three). If an only child feels determined to reverse the pattern, it rather dents the message of enshrining the one-child policy.
Siblings have been close bonds and they have been rivals. Having siblings has a real impact on your character.
It's striking that most leaders in Irish political life have come from bigger families and the interaction with their siblings often helped in the development of leadership.
An exception was Eamon de Valera, who grew up as an only child; some would say that Dev's inflexibility and certainty that he was always right derived from having been raised a lone child. (His mother remarried and had a second son. It is a poignant moment in David McCullagh's recent biography when Dev weeps tears of grief after his half-brother, a priest in America, dies in a car accident.)
Siblings are a strong factor in Mary McAleese's story: she was the eldest of nine. Until she married, aged 24, the former president of Ireland shared a bed with one of her sisters and her siblings are a focus of her affections and concerns.
Although times were hard in Belfast's Ardoyne, it really was the extended family that gave people not only support, but a kind of robustness and energy.
Siblings have been rivals - the Cain and Abel story has been replicated in many genres - and they have been partners.
Brothers have invented things together - Orville and Wilbur Wright with the first airplane - and sisters have sung together (the Nolans, and before them the Beverleys and the Andrews), as have entire families, a la Von Trapp and the Corrs.
When men form a bond, they call themselves a "band of brothers": Jack and Bobby Kennedy were just that, one absolutely reliant on the other. But brothers fall out: look at Princes William and Harry.
When women form a bond, they call themselves a "sisterhood". Constance Markievicz cared more deeply for her sister Eva Gore-Booth than for her husband or children.
Although when sisters (or brothers) fall out, the feud can be permanent, possibly because the emotional involvement runs so deep: like Hollywood stars Joan Fontaine and Olivia de Havilland feuding for decades. Ditto writers Margaret Drabble and AS Byatt. The sister-brother relationship is perhaps less prone to rival hostilities.
There have always been only children, for one reason or another, but the template of human relationships has been based on siblings.
The fall in fertility, with the one-child family becoming more common, must undoubtedly change patterns of psychology, and society.
One Planet, One Child lists the advantages of the one-child family (besides helping out the Brazilian rain forest): better "relationship satisfaction" and the benefit to the child that "one child gets more attention".
How will that pan out? A sarky response on social media predicts that the one-child norm will lead to "more spoiled brats that think the world revolves around them because they never had siblings to teach them otherwise".
My son's response struck a more plangent note: "It sounds so lonely."