(D) THE HEADS OF DURESS
There are three types of duress which will operate as grounds of restitution. Of these, the definition of the first two, duress against the person and against property, is not controversial since they have long been recognized and clearly involve the use of illegitimate pressure. The definition of the third type, economic duress, is controversial, however, primarily because this has only been recognized relatively recently and because the line between what is and what is not regarded as legitimate economic pressure is particularly difficult to draw.
(E) DURESS OF THE PERSON
Duress of the person involves actual or threatened unauthorized interference with the person, whether by endangering life, personal safety or liberty. The test of causation which is applicable to this type of duress is clearly the test of contributory cause, as was recognized in Barton v Armstrong,47 a case which involved threats to kill. Consequently, it is sufficient that the duress of the person contributed to the claimant’s decision to transfer a benefit to the defendant without necessarily being a cause but for which the benefit would not have been transferred.
Usually duress of the person will take the form of the defendant threatening to interfere with the person either of the claimant or somebody else unless the claimant transfers a benefit to the defendant. These threats can take one of three different forms. In each case the threat is unlawful, either because it involves a threat to commit a crime or a tort or, usually, both.  
- (1) Threats to kill. It remains unclear whether a threat made by the defendant to kill will only be relevant where the defendant threatens to kill the claimant or whether it is sufficient that the defendant simply threatens to kill some person. The better view is that the threat to kill need not be directed at the claimant, as long as it can be shown to have influenced the claimant’s decision to transfer a benefit to the defendant. So, for example, if the defendant threatens to kill the claimant’s wife and children if he does not pay some money to the defendant, such an extreme threat must have removed the claimant’s freedom of choice as to what to do with the money.
- (2) Threats to injure. If the defendant threatens to injure the claimant, or presumably anyone else, this will operate as a ground of restitution as long as the threat was a contributory cause of the claimant’s decision to transfer a benefit to the defendant.
- (3) Threats to interfere with liberty. Where the defendant unlawfully threatens to interfere with the liberty of the claimant, or presumably anyone else, and this influenced the claimant’s decision to transfer a benefit to the defendant, this too is a form of duress of the person. This is illustrated by Duke de Cadaval v Collins,4 where the claimant had been unlawfully arrested by the defendant on the ground that he owed the defendant ?10,000. This was a fraudulent claim made by the defendant, but, to secure his release, the claimant paid ?500 to him. The claimant then sought to recover the money in an action for money had and received and succeeded, because the arrest had been wrongful and the defendant knew that he had no legitimate claim against the claimant. This is a good example of a case where the defendant actually exerted pressure against the claimant, rather than merely threatening it, because the claimant’s liberty had already been infringed and would continue to be infringed until he paid the defendant. The claimant had paid the money to remove this pressure in circumstances where the money was not due to the defendant, and this was a sufficient reason for restitution to be
-  suggested that the categories of duress are not closed.
-   AC 104.
-  Consequently, there might be an alternative restitutionary claim grounded on the tort rather than unjustenrichment. See Chapter 17.
-  (18 36) 4 Ad and E 858, 111 ER 1006. See also Pitt v Coomes (1835) 2 Ad and E 459, 111 ER 478.
-  Duke de Cadaval v Collins (1836) 4 Ad and E 858, 864; 111 ER 1006, 1009 (Lord Denman CJ). In fact,Lord Denman held that title to the money never passed to the defendant, so he regarded this as a proprietaryrestitutionary claim. See p 577, below. See also Clark v Woods, Smith and Cooper (1848) 2 Ex 395, 154ER 545.