Population growth. (Think transports mainly, at this point.)
< 19% (and maybe 19% as well) = 10% loss per turn.
>19/20% (20/21% and up) = no growth
>40% (41% & up) = normal growth.
>74% (75% and up) = 1.25x growth.
100% = 2x growth.
Technically, you should have no problem, after your pop has grown to its potential, with running an empire at 41% (or even 21% if you really wanted to push it, but it's nice to have some room to grow if necessary, as well as the fact that at some point it's simply not worth taxing any more). I tend to run mine more so in the ~60% range, which allows me to win elections with relative ease (although sometimes it IS close), while not being quite as difficult to pull off as the 75% necessary for a 25% growth bonus.
Additionally, when looking at empire-wide morale, it helps to take into account that certain planets will be higher or lower (low PQ planets, particularly those low enough to not reach their farm cap, and then there's the civ capitals), so to keep your planets in line with the 41% above you'd probably run around a 46% or 47% empire-wide.