Motives of espionage against ones own country in the light of idiographic studies

The money is perceived as the common denominator among people who have spied against their own country. This assumption is common sense and appears to be self-evident truth. But do we have any hard evidences to prove the validity of such a statement? What method could be applied to determine it? This article is a review of the motives behind one's resorting to spying activity which is a complex and multifarious process. I decided to present only the phenomenon of spying for another country. The studies on the motives behind taking up spying activity are idiographic in character. One of the basic methodological problems to be faced by the researchers of this problem is an inaccessibility of a control group.