In the case of a contract malum in se, it is said, no difference in degree of delinquency will be recognized. Thus, one who hired another to commit murder would not be allowed to recover the money paid, even though induced to contract by the false representations of the assassin. But where a contract is malum prohibitum only, the court may undertake to discriminate between the major and minor offender. This distinction is well put by Wilde, J., in Lowell v. Boston and Lowell Railroad Corporation:1
1 Harse v. Pearl Life Assurance Co.,  1 K. B. 558, (insurance illegal for want of insurable interest).
"In respect to offenses in which is involved any moral delinquency or turpitude all parties are deemed equally guilty, and courts will not inquire into their relative guilt. But where the offense is merely malum prohibitum, and is in no respect immoral, it is not against the policy of the law to inquire into the relative delinquency of the parties, and to administer justice between them, although both parties are wrongdoers." 2
Accordingly, where the contract is malum prohibitum and the plaintiff is not in pari delicto, restitution will be enforced;3 and this is true whichever party first repudiates the illegal transaction and whether or not the illegal purpose of the contract has been accomplished.4
11839, 23 Pick. (Mass.) 24, 32; 34 Am. Dec. 33.
2 See also Smith v. Bromley, 1760, 2 Doug. 696, (money paid to secure signature of creditor to bankrupt's certificate); Kitchen v. Greenabaum, 1875, 61 Mo. 110, (lottery ticket); Tracy v. Talmage, 1856, 14 N. Y. 162; 67 Am. Dec. 132, (bank's illegal purchase of stock).
3 Browning v. Morris, 1778, Cowp. 790, (premium paid for insurance of lottery tickets); Wenninger v. Mitchell, 1909, 139 Mo. App. 420; 122 S. W. 1130, (marriage brokage contract); Tracy v. Talmage, 1856, 14 N. Y. 162; 67 Am. Dec. 132, (bank's illegal purchase of stock). See also cases cited in following sections.
4 Professor Keener treats of the obligation to make restitution where the parties are not in pari delicto under the caption: "Obligation of a Defendant in Default under a Contract unenforceable because of Illegality." This suggests that the doctrine may be invoked only when the defendant has refused or failed to perform; an implication which is followed by the express statement ("Quasi-Contracts," p. 275) that "it is necessary for the plaintiff to show a failure of consideration, and this he can do only by showing that the defendant has failed to perform a contract in exchange for which the benefit was conferred by the plaintiff." In many of the cases hereinafter cited, however, there is no evidence that the defendant was in default.
The cases in which the plaintiff has been declared not to be in pari delicto may all be included, it is thought, within two classes, as follows:
1. Where the prohibition of a statute, or of the common law, under which the contract is declared illegal, is aimed at the act of the defendant and not at that of the plaintiff.
2. Where the plaintiff appears to have been induced to enter into the illegal contract by fraud, duress, oppression, or unfair dealing.