Once again, Ireland is going to the polls about divorce – this time, to decide if separating couples have to wait for four years to be granted a divorce, as is currently the law; or whether this should be reduced to two out of the previous three years. It's time to look again at the state of marriage and divorce.

First, let’s zoom out to get a satellite view of the big trends shaping marriage and divorce in the 21st century. In less than 100 years, what defines a successful marriage has been transformed. Once, if a husband and wife each filled their side of the provider-caretaker bargain, the marriage was deemed successful, although not necessarily happy.

Then, love, affection, romance, companionship and friendship became essentials. Later, more equal sharing of provider and caretaking roles joined the equation. The latest iteration has crept up on us more recently: the belief that a partner should be a soulmate, who shares the other’s dream about their life, helps them make space for it, and encourages them to develop that part of themselves that animates it. That way, each partner keeps growing individually, as does the couple’s relationship.

It’s a noble aim, and a tall order. The result is what’s been called the “all-or-nothing marriage”. The best marriages – or indeed long-term relationships, if the couple decides not to marry – are becoming more personally fulfilling than ever before, and an “average” marriage is now less satisfying. Moreover, whether your marriage or relationship meets your high expectations is now more strongly determining of your physical and psychological wellbeing than ever before.

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This desire for a marriage in which you can grow and develop is not some psychological candyfloss. It runs deeper than that, reflecting more general changes in the life course. We are all living longer. Assuming a couple stay together for the long haul, “till death do us part” is now much further away than it used to be. At age 50, you may have another 30 years to go – a delightful prospect if you are happy; not so great if you’re not.

The notion that you have only one life, and a right to happiness in that life, has taken an irreversible hold on our consciousness. So, unhappy marriages are now more likely to be ended. There are variations in the divorce rate from time to time, but in most developed economies, it is running above 40 per cent. With rates that high, divorce has become a predictable life crisis for many people.

The most common time to divorce is in your mid-40s. But in the US, the divorce rate for those in their 50s and 60s has doubled over the past few decades, with one in four divorces involving people aged 50 and older – the so-called grey divorce revolution.

That’s the big picture. Now zoom back into the micro-world of an individual marriage, and it’s clear that we are a long way from Gwyneth Paltrow-style “conscious uncoupling”. Dashed hopes create the same heartbreak as they always did. Love can be lost quickly enough, but the attachment bond that couples form takes a much longer time to unwind, leaving partners swinging in the wind, unmoored from their once secure base.

Health, concentration and work all suffer. The sleep quality of newly separated people is often as poor as it is in those suffering from a major depression, increasing the risk of higher blood pressure months later. But if women suffer more from the negative effects of an unhappy marriage, the negative effects of divorce hit men more. This period of acute distress, while intense, is generally relatively short-lived, and all but a minority – about 10 per cent – recover in time.

A distressed marriage is not so hot either. The unparalleled physical and psychological health benefits of marriage apply only to couples who are happy and generally satisfied with their relationship. A chronically unhappy marriage starts a cascade of stressors and depression that sets the stage for significant physical and psychological vulnerability, with women at higher risk than men.

Women are more emotionally reactive to negative interactions with a partner, and have more detailed and vivid memories of conflicts. They spend more time thinking and ruminating about the state of their marriage. That’s why relationship-related distress and depression are more closely tied to stress-related illnesses in women than in men.

And then there are the children. Most parents in a chronically unhappy marriage struggle to protect their children from its more pernicious effects. Yet, most children would prefer their parents to stay together, and many parents do for that reason. Either way, there’s a psychological toll. Most children eventually adapt to the breakdown of their parents’ marriage, and become reasonably well adjusted – but “eventually” can span a large part of their childhood – all the years in between playing with Barbie and Transformers to early adolescence.

If the cumulative physical and psychological distress and harm that come from unhappy marriages and divorce were coming from any other source, like pollution or some toxin, there would be public uproar if it were not taken seriously as a public health issue. Yet, it’s not.

Apart from regulating the laws governing marriage and divorce, the whole business is regarded as a couple’s own private business to sort out. Any psychological preparation for marriage is left to church-based volunteers who do their best to share the lessons that they have learned from their marriages. Inevitably, it’s a hit-and-miss process.

So what does all this mean for the future of marriage and divorce?

The high probability is that most people will still expect, want, and end up getting married. Couples will still walk up the aisle with high hopes of happiness and of staying together for the rest of their lives – albeit with a silent proviso of “unless things become intolerable”. So the breakdown of marriages is also here to stay.

Divorce rates are very low in Ireland, at 4.7 per cent (of people who were ever married) in the 2016 census. But what happens in other countries often has a way of lapping up in less extreme forms on our shores. Being an intimate and generally family-oriented society and, at least nominally, Catholic, may keep the divorce rate relatively low here. But so too is Spain, and the divorce rate there is 65 per cent. So we can expect some increase here.

The more pressing question is what can be done to prevent the distress of a chronically unhappy marriage, or the breakdown of a relationship. It’s clear that by the time a couple separates, a lot of damage has been done. So, making them wait for four years for a divorce in the hope of a reconciliation is very unlikely to work.

Here is what we should be paying attention to: One of the most consistent and consequential research findings over the past 20 years is that the most robust and early warning signs of trouble are evident not just in the first year of marriage, but before a couple even tie the knot.

Take just one study on 90 newlywed couples, average age 25, selected by Ohio State University on the basis of their exceptional physical and mental health. Over a 24-hour period, each partner's endocrine and immune functioning are intensively monitored. Early in the day, the couple is asked to discuss some disagreement they are having. Some couples, compared with others, communicate in a more negative way as they try to resolve a disagreement. They are more sarcastic. Their tone is more critical or domineering. They get angrier. They roll their eyes, signalling contempt for what the other is saying; nothing very exceptional you might think.

But these markers of hostility trigger an immediate increase in stress hormones. This physiological stress response continues after the discussion ends, and is still evident during the night when they are sleeping. This persisting stress response has downstream effects. It interferes with their endocrine and immune functioning, producing the kind of inflammatory response that raises the risk of heart attack and stroke, metabolic syndrome, diabetes, arthritis, and poorer wound healing. These negative effects are much more marked in women, particularly if their husbands tend to withdraw, or refuse to engage, after a conflict.

This increase in stress hormones in the first year of marriage is three times higher in couples who subsequently divorce than in those who don’t. And if the couples don’t divorce, and are still unhappy 10 years later, their stress hormones have doubled, and their immunity is further compromised.

So, do things get better if the couple sticks it out for the long haul? Short of some effective intervention, the answer is no. Couples who’ve been together for more than 40 years show similar or more intense stress responses to negative interactions. Plus, by now, the conflicts are likely to be more frequent and intense, and the adverse effects on their health are now becoming obvious.

There is one more effect of living together for a long time in an unhappy marriage. Two become one in a very physical way; their bodies become “entrained”. One partner’s stress response is now in lock step with the other’s. That’s one reason why in a long-term marriage, a couple’s immune systems, gut health and gene expression – how information in your DNA gets converted into proteins that allow you to adapt to changing conditions – begin to converge.

One partner’s increased heart rate, hypertension, inflammatory response, depression and cognitive functioning may become linked to the other’s. As one researcher put it, these findings give a whole new meaning to “Till death do us part”.

Such research points strongly to the need for early, targeted and highly skilled interventions to help couples recognise negative patterns of interaction, and manage them better. Alerting couples to these high-risk behaviours won’t mitigate all the risks that beset marriage. But it will go some way to address the more day-to-day, mundane, but powerful risks that creep up on even the most loving couples.

The simple and most powerful message to couples is this: Love, attraction, romance, interest, fun, friendship all count. These positives are what draw couples together. But they don’t predict how a marriage will turn out. The most powerful predictor of an unhappy marriage is how much negativity is expressed when trying to resolve a disagreement, and this pattern of negative communication, without intervention, will prove to be remarkably enduring.

Negatives and positives are not equal opportunity emotions. Negatives have five times the impact that positives do. That’s why it takes five times as many positive interactions than negative to absorb the wear and tear of close-up living.

Negatives erode positives over time, unless a couple work hard to balance that out with very high levels of positivity as they negotiate the tricky waters of the early stages of a relationship. They have to be alert to any decrease in positivity between them, and make strenuous efforts to put things back on track.

Bad is stronger than good. Negatives are a much more powerful risk factor, than positives are a protective factor. This does not mean that positives don’t matter – they do. But negatives matter more.

So, pay heed young lovers: Primum, non nocere. First, do no harm.

Dr Maureen Gaffney is a clinical psychologist and writer. Her new book, Your One Wild and Precious Life, will be published by Penguin later this year.