From 'The Scientology Handbook'

There are ways, however, not only to select a marriage partner, but also to guarantee the continuation of that marriage; and these ways are simple. They depend uniformly upon communication.

There should be some parity (equality) of intellect and sanity between a husband and wife for them to have a successful marriage. In Western culture, it is expected that the women shall have some command of the humanities (cultural studies such as language, literature, philosophy and art) and sciences. It is easy to establish the educational background of a potential marriage partner; it is not so easy to gauge their capability on the second dynamic or their sanity. 

In Scientology, there is a test for sanity and comparative sanity which is so simple that anyone can apply it. What is the "communication lag  of the individual? "Lag" means an interval between events. When asked a question, how long does it take him to answer? When a remark is addressed to him, how long does it take for him to register and return? The elapsed time is what is called the communication lag. The fast answer tells of the fast mind and the sane mind, providing the answer is a sequitur – something following logically; the slow answer tells of less ability and sanity. Marital partners who have the same communication lag will get along; where one partner is fast and one is slow, the situation will become unbearable to the fast partner and miserable to the slow one. Further, Scientology when applied will be more swiftly active in the case of the fast partner and so the imparity under processing will grow beyond either's ability to cope with the matter.