We were reminded of this legal concept in the recent case where Vicky Pryce deployed it in her defence to a charge of perverting the course of justice. She claimed that her former husband, Chris Huhne, a cabinet minster coerced her into falsely claiming that she had been driving a speeding car, so allowing him to avoid prosecution and keep his driving licence.
We don't yet know if this defence will succeed. The jury in the case was unable to come to a verdict and there will be a retrial.
But what of this "Marital coercion" defence? Men cannot use it, only women. This is curious. You could see it as a protection women have against overbearing husbands, the patriarchal society, etc. On the other hand, surely it is demeaning to women to suggest they can be pressured in a way that men cannot? And can a man never be pressured by his wife? Perhaps a more understandable aspect is that the defence is not available to a Civil Partner - they hadn't been heard of in when the Criminal Justice Act 1925 was passed ("but on a charge against a wife for any offence other than treason or murder it shall be a good defence to prove that the offence was committed in the presence of, and under the coercion of, the husband.")
To my mind, any true supporter of gender equality should be campaigning for this anomaly to be removed, by which I mean that the "Marital coercion" defence should be abolished.