The intellectual functioning of SO married couples, each having 1 psychiatric day hospital patient-spouse, is described and evaluated by the use of Interaction Testing, a technique which includes the testing of the individual spouses and the married pairs on a set of intelligence test problems. Marital intelligence is denned as the ability of the couple to solve these problems and is measured by the IQ score achieved by the couple as a unit. The reliability of Interaction IQ test scores, the nature of husband and wife contributions to marital "intelligence," and the reliability of a scoring system of interaction process or decision making inferred from a comparison of "couple" vs. individual products are considered. Interaction Testing appears to be a feasible and reliable technique for the investigation of marital intelligence and decision making, and offers to be of use in family diagnosis and research.