It's the same as population improvements/resources; It multiplies the current modifier if it is not a same.
Example: Say you have Fish (+8%), two Clinics (2%+2%= +4%), a Hospital (+6%) and a National Research Lab (+5%).
1.08x1.04x1.06x1.05= 1.25 population modifier.
Afaik the population boosts aren't as great as the +5 happiness/+$10 wonders on average (except for the DRA, that provides another vital aid slot).