Quote:
Originally Posted by DSouth
I don't understand the intuition behind the last sentence which I've copied below:
MARSTAT is an indicator for whether the person is married. AGE is the age of the primary person. SAGE is the age of the spouse, SEDUCATION is the spouse's education.
Thanks guys!

This is the only interpretation that made sense to me: "What this tells us is that if any of the three variables are used (SAGE, SEDUCATION, or AGEdiff), we must also include MARSTAT."
The intuition is that if you use any of the variables that relate to a characteristic of a spouse, you have to also include the marriage status.