People today think they know the 'facts of life', but to judge from a new survey of attitudes to life, love and happiness, nothing could be further from the truth.
Last week Margaret Fine-Davis of Trinity College Dublin published an excellent new study called 'Attitudes to Family Formation in Ireland'. It is based on a survey of 1,400 people aged 20-49.
One of its findings is that 84pc of people in this age group believe it is better to live with someone before you marry them. But they are quite wrong about this.
If cohabitation is good for marriage, then how do they explain the fact in that in countries where cohabitation is widespread and has been for a long time, like Sweden or Britain or America, the divorce rate is high and the marriage rate low?
In addition, 69pc of respondents think cohabitation provides just as strong a basis for family life as marriage. That's not true either.
The British Millennium Cohort Study found that cohabiting parents break up twice as much as married parents.
Fifty-six per cent believe that one parent can bring up a child just as well as two parents together. This can be true in individual cases, but it's not true in general. Raising a child on your own is much harder than raising a child with another person.
To begin with, single-parent families are far more likely to be poor, and as the report itself shows single mothers are the most likely to experience loneliness. This stands to reason.
How can we explain such a disparity between what people believe and what is actually true? I think it arises from a combination of wishful thinking and a reluctance to cause offence.
Because cohabitation is now so widespread, we convince ourselves that cohabitation is good for marriage.
And because we don't want to offend anyone (least of all ourselves), we say that children do just as well whether their parents are married or cohabiting or living without another adult, regardless of the evidence.
Another very good study about the family was also launched this week.
Written by Tony Fahey and Peter Lunn and published by the ESRI, it examines family structure in Ireland today based on data from Census 2006.
It confirms that the Irish family is changing rapidly. The main finding is that a third of Irish families now fall outside the traditional model of a man and woman in their first marriage.
Over at the 'Irish Times' the other day, Carl O'Brien -- an otherwise excellent reporter -- advised us all to relax about this.
He accepted that the traditional family based on marriage is the gold standard family unit, "given the positive outcomes for children in these settings".
But then he said that because this family still accounts for two-thirds of family units in Ireland, rumours of its demise are exaggerated.
On the basis of this logic, though, we should also calm down about unemployment because 85pc of people are still in work, or about poverty because the vast majority of us are above the poverty line.
But no-one ever says either of these things because an unemployment rate of 14.5pc accounts for an awful lot of people in absolute terms, and likewise the poverty rate of 15.8pc.
In terms of the family, a quarter of children, or almost 300,000, are raised outside the marital family, which is to say outside the "gold standard family unit", as Carl O'Brien puts it. That is a lot of children.
Obviously lots of lone parents and lots of cohabiting parents do a very fine job raising their children and many do a better job than married parents.
But since the marital family is the "gold standard family unit", a figure of 300,000 should have our alarm bells ringing in the same way a 14.5pc unemployment rate and a 15.8pc poverty rate have our alarm bells ringing.
We should be working to strengthen and promote marriage if children are really at the heart of family policy and if we believe children ought to be raised by their own mothers and fathers.
Several factors are driving up the number of children who don't have the advantage of being raised in the "gold standard family unit".
One is separation and divorce. Another is the number of children born to lone parents. Another is cohabitation which is not the same as marriage because, as mentioned, cohabiting parents are far more likely to break up than married parents.
In fact, the number of children being raised outside of marriage in Ireland today is now high compared to a lot of other European countries, so the family in Ireland is not in rude good health, far from it.
But we remain almost entirely complacent about this. When it comes to the family, we prefer to tell ourselves comforting lies rather than face up to the facts of life.