Aresearch study indicated that the more work-related stress one feels then the less satisfied they feel about their marriage. this finding illustrates a correlation between people's work-related stress and their marital happiness.

I think the correct term to fill in the blank would be "negative". The finding that states "the more work-related stress one feels then the less satisfied they feel about their marriage" describes a negative correlation between people's work-related stress and their marital happiness. A correlation is a measure of dependence of two variables - dependent variable and in dependent variable - in a given system. A negative correlation would mean that the relationship between variable is inverse wherein when one variable increases the other would decrease. The opposite of this is called the positive correlation wherein when one variable increases, the other increases as well.