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In jurisprudence, undue influence is an equitable doctrine that involves one person taking advantage of a position of power over another person. This inequity in power between the parties can vitiate one party's consent as they are unable to freely exercise their independent will.
In contract law
A contract may be seen as undue influence when one party uses undue influence to persuade another party into entering into a contract or the transfer of property which is disadvantageous to the influenced party. If undue influence is proved in a contract, the contract is voidable by the innocent party, and the remedy is rescission. There are two categories to consider:
- Presumed undue influence
- Actual undue influence
Presumed undue influence
In the first subgroup, the relationship falls in a class of relationships that as a matter of law will raise a presumption of undue influence. Such classes include:
- Government/citizen (note this is not confirmed)
- Religious adviser/member of the flock
- Solicitor (attorney)/client
- Doctor/patient (note this excludes dentist and patient)
In such cases, the burden of proof lies on the first of said parties (e.g. the government, parent, or doctor) to disprove undue influence on the second party. This requires the dominant party to establish that the second party "knew and understood what he or she was doing, and that he or she was acting independently of the influence of the dominant party". One influential factor in deciding whether the second party was acting independently is whether he or she was given an independent advice, while such an advice is not indispensable for rebutting the presumption. 
The second subgroup covers relationships that do not fall into the first subgroup, but on the facts of case, there was an antecedent relationship between the parties that led to undue influence. The test is one of whether "“one party occupies or assumes towards another a position naturally involving an ascendancy or influence over that other, or a dependence or trust on his part". If the plaintiff satisfies this a presumption of undue influence will arise, to which the onus of proof transfers to the defendant, who thereon, must rebut that "in all the circumstances", the relationship between the parties involved "dealings were at arm’s length and that the other’s will was in no way overborne by the relationship of confidence" that existed.
In Garcia v National Australia Bank (1998) 194 CLR 395, the High Court of Australia approved the principle in Yerkey v Jones  HCA 3 by distinguishing between cases of actual undue influence and situations where the transaction is set aside because the guarantor does not understand the nature of the transaction. Although there is no presumption of undue influence, a "lender is to be taken to have understood that, as a wife, the surety may repose trust and confidence in her husband in matters of business and therefore to have understood that the husband may not fully and accurately explain the purport and effect of the transaction to his wife; and yet... did not itself take steps to explain the transaction to the wife or find out that a stranger had explained it to her."
Actual undue influence
An innocent party may also seek to have a contract set aside for actual undue influence, where there is no presumption of undue influence, but there is evidence that the power was unbalanced at the time of the signing of the contract., Factors such as age, mental capacity and literacy of the donee, among other considerations such as the nature of the transaction (fair or unfair) will help determine actual undue influence. There is no requirement of manifest disadvantage.
In probate law
"Undue influence" is the most common ground for will contests and are often accompanied by a capacity challenge. That is, someone in possession of full mental capacity is not likely to be swayed by undue influence, manipulation, or coercion. In litigation most jurisdictions place the burden of proving undue influence on the party challenging the will. Undue influence can be very difficult to prove, and the mere appearance of undue influence is inadequate to challenge the validity of a will.
In probate law, undue influence is generally defined as a testator's loss of free agency regarding property disposition through contemporaneous psychological domination by an advisor, resulting in an excessive benefit to the advisor. It is important to note that "undue influence" is only an issue when the advisor is benefiting, not when advisor is getting a benefit for someone else; in that case it would be considered fraud.
- Compare with duress
- Johnson v Buttress  HCA 41  AustLII
- Commercial Bank of Australia Ltd v Amadio  HCA 14; (1983) 151 CLR 447 AustLII
- Farmers' Co-Op Executors & Trustees v Perks (1989) 52 SASR 399
- R v Attorney General for England and Wales
- =Bainbrigge v Bowne (1881) 18 Ch D 188 at 196; London and Westminster Loan and Discount Co Ltd v Bilton (1911) 27 TLR 184; see also West v Public Trustee  SASR 109 AustLII
- Johnson v Buttress  Dixon J, High Court of Australia
- Powell v Powell  1 Ch 243
- Allcard v Skinner (1887) 36 Ch D 145; See also McCulloch v Fern  NSWSC 406 AustLII, Hartigan v International Society for Krishna Consciousness Incorporated  NSWSC 810 AustLII
- Re P's Bill of Costs (1982) 45 ALR 513 at 521-5; Westmelton (Vic) Pty Ltd v Archer and Shulman [1983 VR 305] AustLII;
- see also Haywood v Roadknight  VLR 512 AustLII
- Dent v Bennett (1839) 4 My & Cr 269; 41 ER 105; Williams v Johnson  4 All ER 34
- Brooks v Alca (1976) 60 DLR (3d) 577
- Tulloch (deceased ) v Braybon & ors (No 2)  NSWSC 650,  AustLII
- see also Watkins v Combes  HCA 3; (1922) 30 CLR 180 AustLII
- Inche Noriah v Shaik Allie Bin Omar  AC 127BAILII
- Johnson v Buttress  HCA 41 AustLII
- Westmelton (Vic) Pty Ltd v Archer and Shulman  VR 305 AustLII
- Garcia v National Bank Australia Ltd 194 CLR 395  AustLII
- see Commercial Bank of Australia Ltd v Amadio  HCA 14; (1983) 151 CLR 447 (12 May 1983) http://www.austlii.edu.au/cgi-bin/disp.pl/au/cases/cth/HCA/1983/14.html?stem=0&synonyms=0&query=amadio AustLII,
- see also Westmelton Pty Ltd v Archer and Schulman  VR 305 http://www.austlii.edu.au/cgi-bin/sinodisp/au/cases/vic/VicRp/1982/29.html?stem=0&synonyms=0&query=title(Westmelton%20and%20Archer%20)
- See Justice Starke in Johnson v Buttress  HCA 41  AustLII
- See Blomley v Ryan  HCA 81; (1956) 99 CLR 362 (28 March 1956) AustLII
- Core v. Core’s Administrators, 124 S.E. 453 (Va. 1924).
- Time Limit and Grounds for Contesting a Will. Accessed May 15, 2015 - Going Legal Limited.
- Ronald J. Scalise Jr., Undue Influence and the Law of Wills: A Comparative Analysis, 19 DUKE J. COMP. & INT’L L. 41, 99 (2008).