How can I tell whether behaviour is ‘unreasonable’?

There is no easy answer to this. Some cases are very straightforward, like violence, for example. In many cases it is a question of the particular effect the behaviour of one partner has on the other. If the effect is mild (irritation, annoyance, boredom) it is not sufficient. If the effect is more dramatic and this is clear to your partner, it may well be sufficient. The test is whether he or she has behaved in such a way that you cannot reasonably be expected to live with him or her. The question the judge asks himself is: ‘Would a reasonable person conclude that this person cannot be expected to live with his or her partner?’

Example 1: Rob tends to squeeze toothpaste from the middle of the tube. Pippa is a very orderly person and this annoys her.

A reasonable person would probably conclude that this was part of the wear and tear of living with someone and that it was not unreasonable to expect Pippa to put up with it.

Example 2: Pippa had a traumatic childhood experience involving toothpaste being squeezed from the middle of the tube. She has told Rob about this and asked him repeatedly not to squeeze the toothpaste like that because it triggers memories and makes her feel stressed and unhappy. Rob deliberately continues to squeeze the toothpaste from the middle of the tube.

Here, a reasonable person might conclude that it is unreasonable to expect Pippa to continue living with Rob because Rob's behaviour has a very strong negative effect on her. Rob knows the effect it has and could easily change his behaviour but deliberately chooses not to.

In general a number of different types and incidents of unreasonable behaviour are set out in petitions. It is important that at least one of these has taken place within six months before starting divorce proceeding, otherwise, as in adultery, you’re presumed to have forgiven your partner.