5. studies indicate that married men on average earn more income than unmarried men of the same age and education level. why must we be cautious in concluding that marriage is the cause and higher income is the effect

From this statement, we can conclude that there is a correlation, but marriage is not necessarily the cause of higher income for married men.
Rather than the marriage itself, the higher income could be caused by commitment.
We can conclude that men who are married show the tendency to commit to one thing. If this tendency is brought to their career, they will much more likely to get higher income.