THE mid-19th century saw the emergence of the idea that individuals should be left free to choose their own actions subject only to the condition that their choices imposed no harm on the rest of society.
The pro-abortion result in the recent Irish referendum and the state of much public opinion in Britain demonstrates how prevalent that mindset has become.
There is, however, a major problem with radical individualism or libertarianism: very few actions are without some wider social consequence.
Sometimes those consequences are negative. Freer access to abortion throughout the Western world may have had one of the most profound social and economic consequences imaginable - fewer children being born. The tendency in these countries is now towards a shrinking population.
In Britain, as in Ireland and much of continental Europe, what demographers call the 'total fertility rate' has for some time now been well under the critical value of 2.0.
The fertility rate in Britain collapsed from about 2.9 at the start of the 1960s to only about 1.7 in the late 1970s.
That change coincided with the introduction of the contraceptive pill and de facto abortion on request.
Even placing to one side wider legal, scientific or moral considerations, abortion is not just a matter of individual choice
The total fertility rate is the average number of children born or likely to be born to a woman across all her child-bearing years.
Once that figure falls below about 2.1 then without some other major change - notably immigration - the population will be reduced in the long-run.
During the 1960s-1970s and again in the 2000s, most of the Western countries compensated for declining fertility through sizeable immigration.
That meant that as the 'indigenous' population aged and then began to shrink there were still enough people of working age to pay the taxes to fund spending on pensions and other social services.
However, it is clear that we are in the middle of a public opinion and electoral backlash against immigration.
What we may be observing is an inconsistency or incoherence to the extent that people are demanding both freer abortion - almost certainly accompanied by declining fertility - and less immigration.
It is striking that Hungary is one European country which has combined resistance to open borders with relatively pro-family tax and welfare policies; the Hungarian abortion rate has declined markedly since 2010.
Whatever one thinks about all the aspects of the politics of the Hungarian prime minister Viktor Orbán, there may be some recognition in Hungary that social choices have to be coherent.
Northern Ireland's fertility rate is currently about 2.0 compared to a UK average of about 1.8.
In 2017 the Northern Ireland Statistics and Research Agency published a range of demographic projections through to 2045.
One of those projections indicates what would happen to Northern Ireland's total population if fertility declined from its current relatively high level to the lower level in Britain: the population level in 2045 would be 100,000 smaller than it would otherwise have been, with all other factors remaining constant and compared to a projected population of about two million.
Of course, many factors impinge on fertility but the ease of access to abortion must be a major factor - in 2017, the Republic's fertility was already 1.8 but it is likely the recent referendum decision would lead to that dropping still further.
Adoption of liberalised abortion laws make it much more likely fertility rates in Northern Ireland will converge to the levels currently experienced in Britain and in turn make it likely the total size of population will be substantially lower.
This is an argument about future trends and so is different from the point previously made by 'Both Lives Matter' - based on a set of reasonable assumptions, it is likely that about 100,000 are alive in Northern Ireland in 2018 who would not have been if Northern Ireland abortion rates had followed the pattern set by Britain since 1967.
Adoption of liberalised abortion laws make it much more likely... the total size of population will be substantially lower
In summary, even placing to one side wider legal, scientific or moral considerations, abortion is not just a matter of individual choice.
Liberal abortion policies tend to be associated with populations that are either smaller or older, or both.
By backing freer abortion the baby boomers and subsequent generations have intensified the challenge of paying for their own upkeep in retirement.
This is especially so given that it can no longer be assumed that substantial immigration can be the back-stop policy option.
Economics has, after all, quite a lot to say about abortion policies. For example, George Akerlof and Janet Yellen - the former a Nobel prize winner and the latter a one-time chair of the US Federal Reserve - published an article in 1996 on some of the social consequences of abortion liberalisation.
Population growth has long been an issue of concern for economists; 220 years ago, Thomas Malthus, an Anglican clergyman as well as an economist, argued that there was a tendency for population numbers to outgrow the supply of food.
The result, he said, would be poverty, misery and death. A great irony is that today we face the reverse social and economic challenge: too few babies are being born.
- Dr Esmond Birnie is an economist and a former Ulster Unionist MLA.
