(establishing the ground of divorce)the ground of divorce, irretrievable breakdown, is established by this mode, in English and in Scots law, if the defender has at any time during the marriage behaved (whether or not as a result of mental abnormality and whether such behaviour has been active or passive) so that the pursuer cannot reasonably be expected to cohabit with the defender. The way in which the mode is set out makes it evident that the focus is at the time of the petition, and accordingly there is always the argument that aged incidents have ceased to have effect, for the pursuer has subsequently found it reasonable to live with the defender despite the incident. Thus, courts look at the whole circumstances and often seek a pattern of behaviour in which older incidents are highly relevant. A single incident, however, can constitute behaviour if it strikes at the heart of the marital bond.
Collins dictionary of law. W. J. Stewart. 2001.