This section is from the book "The Law Of Contracts", by William Herbert Page. Also available from Amazon: Commercial Contracts: A Practical Guide to Deals, Contracts, Agreements and Promises.
It has become the established doctrine, according to the weight of authority at modern law, that the test for duress is not whether a courageous man, or even a man of ordinary firmness, would have been overpowered by the duress, but whether the specific individual against whom the duress was exerted was in fact overpowered.1 So in fraud the modern tendency is to hold that the test for fraud is not whether the representation would have deceived a man of unusual shrewdness, or whether it would have deceived a man of ordinary shrewd-ness, but whether it did deceive the specific individual who is made the victim of such fraud.2 The application and extension of this doctrine result in the doctrine of undue influence.
1See for the meaning of "undue influence":
England. Smith v. Kay, 7 H. L. Cae. 750.
United States. Sturm v. Stump, 239 Fed. 7*40.
Alabama. Frederick v. Hartley, - Ala. - , 79 So. 381.
Illinois. Roberts v. Weimer, 227 111. 138, 81 N. E. 40.
Kentucky. Chrisman v. Quick, 174 Ky. 845, 193 S. W. 13.
Mississippi. Gillis v. Smith, 114 Miss. 665, 75 So. 451.
New Himpshire. Curtice v. Dixon, 74 N. H. 386, 68 Atl. 587.
New Jersey. Lomerson v. Johnston, 44 N. J. Eq. 93, 13 Atl. 8; Colonial Building & Loan Association v. Griffin, 85 N. J. Eq. 455, 96 Atl. 901.
Oregon. Coleman v. Coleman, 85 Or. 99, 166 Pac. 47.
South Carolina. Hugging v. Hug-gins, 107 S. Car. 470, 93 S. E. 129.
2Beard v. Beard, 173 Ky. 131, 190 S. W. 703.
3 "Undue influence is a species of constructive fraud which the courts will not undertake to define by definite words or rules. Influence, to render a conveyance inoperative, must be of such a nature as to deprive the grantor of his free agency. Undue influence means a wrongful influence, such as influence as makes the grantor or testator, in the instrument excuted, speak the will of. another and not his own. It is not sufficient to avoid a will or deed that its execution was procured by honest argument, untainted with fraud. Proper and legitimate influence, honestly acquired, is not the exercise of undue influence." Valbert v. Val-bert, 282 111. 415, 118 N. E. 738.