Lots of reasons are given as explanations: We live in a very different society from 30 or 40 years ago and there is no longer the same expectation or pressure for us to marry. We are no longer governed by religious codes of behaviour that said sex before marriage was sinful, and that children born outside wedlock were illegitimate. The law governing matrimonial property and child custody is now no different for people in de facto relationships compared to those who are married. And so on.
Simply put, the institution of marriage is impacted by social change, and shows up in the stats.
However, once we are in a committed relationship for longer than three years, we are effectively as good as married under the law - unless we take steps to keep our financial affairs separate.
Doing this creates some complex (but usually useful conversations) for couples embarking on a serious relationship, around harder topics like life goals and finances. Sharing a child is also legally no different for married or unmarried parents.
Our laws, then, have adapted to fit social change. But what about the psychology underpinning of all this? Does marriage no longer have meaning? Is it 'bad' for society that marriage rates are on the decrease?
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Being more cautious about jumping into marriage - or swerving away from it altogether - doesn't mean we've lost romantic commitment as a core value. Most of us still want to be loved and to love, for all the same and normal reasons that have drive human psychology forever. Attachment is a fundamental need, not a fashion. We transfer our primary attachment to parents to our adult partnerships. So while the "to have and to hold" bit may not be a governing aspect of family life anymore, that doesn't mean that we take commitment lightly.
Nor does being unmarried to our partner lessens grief and stress when a relationship ends.
Would we work harder at our relationship if we were married? There's no real answer to that. Many people remember their parents' marital conflict and wish they had separated; living in conflict and feeling unable to get out did nothing for family health back in the day when divorce was full of stigma.
On the other hand, a generalised dissatisfaction as a reason to quit a committed relationship ('I still love her/him, but I am not in love anymore') can be heard in counseling rooms throughout the country - including mine. And it's no less difficult for unmarried couples simply because they don't have to formally separate in the eye of the law.
Those who do wish to marry - whether by traditional heterosexual white wedding ceremonies, civil unions, same sex marriages or quiet registry office affairs - often want to make a public statement to friends and family. Other couples aren't interested in public statements.
Why are we interested in the public statements of the likes of Kim Kardishian - or the conscious "uncoupling" of Gwyneth Paltrow and Chris Martin? Because love and marriage and pain and divorce are the yardsticks of our human experience, and we can see ourselves in the details. Whether we pour over stories in tabloid magazines, the local bar or the school gates, stories give us a comparison with which to measure and make sense of our own experiences.
Ultimately, the real challenge is not whether we should or should not marry, but how to live our lives with integrity as well as freedom - especially within committed relationships. It can be a complicated balancing act. Perhaps this is why we sneak a look at how others are doing it as we continue to navigate our own personal paths.