Twitter behavior can predict users' income level

> for example, those who earn more tend to express more fear and anger on Twitter.

That is to be expected. If you know your r/K selection theory, K individuals have larger-than-average amygdalae which is responsible for awareness, risk-assessment and consequentially fear.

K individuals in human beings are those who aren't promiscuous, marry early and have kids, and by consequence are infinitely less likely to be poor.

> Perceived optimists have a lower mean income.

This also makes sense, since r-individuals in human beings are more welcome to hope and change (optimism), and they tend to be less well-off financially since they switch their survival needs onto K-individuals (demanding help from them for paying their student loans, providing food stamps, etc).