'Unreasonable behaviour' is the most common ground that married couples cite to legally end their unions, a study conducted in the UK has found.
Researchers from the University of Oxford analysed the changes in the main 'facts' that husbands and wives give for petitioning for divorce, since the UK Divorce Reform Act 1969 was implemented in 1971.
The Act made the irretrievable breakdown of a marriage the sole ground for divorce, which can be established by 'proving' one or more 'facts' - though the 'fact' is not necessarily the cause of the breakdown.
In addition to 'fault-based' facts, which imply blame: 'unreasonable behaviour,' adultery and desertion, the Act introduced two new innovative, 'no-fault' separation facts; one of two years' separation with both parties agreeing to divorce, and the other of five years' separation without agreement.
It was originally hoped that these 'no-fault' facts would account for most divorces, but, despite an initial uptake, this has not proven to be the case, researchers said.
Over time, people's use of the law for legally ending their unions has changed considerably, with the fault-based fact of 'unreasonable behaviour' most used in recent years, and desertion the least, they said.
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John Haskey, an Associate Fellow at Oxford, found that after 1971, the facts used most frequently to get a divorce were 'fault' based - 'unreasonable behaviour' for wives, and adultery for husbands.
However, since 1991 there has been a modest growth in the relative importance of both the separation facts, for divorces granted to both husbands and wives, researchers said.
Divorces granted to wives citing adultery peaked in 1987 at 25 per cent, and at 45 per cent of those awarded to husbands, they said.
However, both proportions fell substantially to 11 per cent in 2016. In the case of husbands, this trend was counterbalanced by the growth in the proportion of divorces on 'unreasonable behaviour'.
The proportion of divorces granted on the grounds of 'unreasonable behaviour' has grown considerably, according to the researchers.
Of divorces granted to wives, the proportion awarded on 'unreasonable behaviour' has trebled from 17 per cent in 1971 to 51 per cent in 2016, they said.
For divorces granted to husbands, the corresponding change has been even more dramatic, from having been the least used fact in 1971 to the most used in 2016 (from two per cent to 36 per cent of husbands' divorces).