Both, and income is often measured as a function of family size (e.g., poverty line for singles is lower than for families). In other words: lower income folks

Both, and income is often measured as a function of family size (e.g., poverty line for singles is lower than for families). In other words: lower income folks are more likely to have kids, more likely to have more kids, and each subsequent kid lowers relative income (according to most income metrics).