In Pennsylvania, undue influence is described in the leading case in the Commonwealth by its Supreme Court in Estate of Clark, 461 Pa. 52, 334 A.2d 628 (1975).  The Court set forth therein that where (1) a person who is in a confidential relationship with the testator, (2) receives a substantial benefit under the proposed will, and (3) at or around the time the Will was executed the testator had a weakened mental intellect, a presumption of undue influence arises.

A confidential relationship exists when "the circumstances make it certain that the parties did not deal on equal terms, but, on one side, there was an overmastering influence or, on the other, weakness, dependence or trust, justifiably reposed.  Leedom v. Palmer, 274 Pa. 22 (1922).

Examples of confidential relationships are presumed between attorney and client, guardian and ward, trustee and beneficiary and may be found as between doctor and patient, nurse and patient, accountant and client and banker and client.

A weakened intellect is a relative term since it contrasts the relative intellect of the undue influencer and the testator.  It is " a mind in which, in all the circumstances of a particular situation, is inferior to normal minds in reasoning power, factual knowledge, freedom of thought and decision, and other characteristics of a fully competent mentality."  Paolini Will, 13 Fiduc. Rep. 2d 185 (O.C. Montg. 1993).  However, a weakened mental intellect need not rise to the level of a lack of capacity.  Estate of Lakatosh, 441 Pa. 133 (1995).

Pennsylvania wills may also be attacked on grounds of forgery, mistake, fraud or lack of testamentary capacity or insane delusion by the testator.  Also, a will may be challenged for entry to probate by questioning whether the document was properly executed.

To prove fraud, contestant must show 1) decedent had no knowledge of the concealed or misstated fact and 2) the decedent would not have made the same bequest had he known the truth.  Estate of Paul, 407 Pa. 30 (1962).

Forgery can be shown where there is an authorized signature of the decedent by another, a scheme of disposition of the estate written above a disconnected signature of the testator or pages of a will have been substituted therein.  Kane's Estate, 312 Pa. 531 (1933).

Mistake in the execution of a will can be found in two forms:
1) mistake in executing a document not knowing it was a will or 2) testator was induced to execute the will by a factual mistake or mistaken belief as conveyed to him by another.