(A) THE RATIONALE OF DURESS AS A GROUND OF RESTITUTION
Essentially, duress operates as a ground of restitution where the claimant has transferred a benefit to the defendant as a result either of the exercise of illegitimate pressure or the making of an illegitimate threat. This threat may either be made expressly or implied from surrounding circumstances, but it must take the form of ‘do this or else... ’.
The reason why a claimant who succumbs to the illegitimate threat or pressure by transferring a benefit to the defendant should be able to obtain restitution is usually considered to be that the effect of the threat or pressure is to vitiate the claimant’s intention that the defendant should receive the benefit. But this notion that the duress vitiates intent is not strictly accurate, since the claimant always has a choice whether or not to transfer the benefit to the defendant; the claimant does not need to submit to the threat or the pressure, but chooses to do so. The real reason, therefore, why duress should be recognized as a ground of restitution is simply that the effect of the duress is that the claimant cannot be regarded as having exercised a free choice when he or she transferred a benefit to the defendant. In other words, the duress interferes with the claimant’s autonomy to benefit whomsoever he or she wishes without constraint. This was recognized by Lord Scarman in The Universe Sentinel: ‘The classic case of duress is... not the lack of will to submit but the victim’s intentional submission arising from the realization that there is no other practical choice open to him.’
Consequently, the claimant’s intention to benefit the defendant can be considered to be vitiated, but only because the claimant did not exercise a free choice to benefit the defendant.
-  Woolwich Equitable Building Society v IRC  AC 70, 165 (Lord Goff).
-  See, for example, Barton v Armstrong  AC 104, 121 (Lords Wilberforce and Simon).
-  This was recognized by the House of Lords in DPP for Northern Ireland v Lynch  AC 653. LordSimon, ibid, 695, said that ‘duress is not inconsistent with act and will, the will being deflected and notdestroyed’.
-   1 AC 366, 400 (Lord Scarman).