Think about it, who has more opportunities to find a good job? Someone who can read and write well, or someone who doesn't know how to read and write well? The correlation between literacy levels and standard of living is very strong. The more education an individual has, the more opportunities they may have to better their life.
Therefore, your best answer is "When one social condition rises or falls, so does the other. Literacy rates are positively correlated with standard of living."