Newbie Questions on Research vs Caps

[xNECROx]

Is the "empire wide cap" the total of all 3 research fields or just 1? What affects the research cap? Is it population? Happiness? In my human game with just my main colony, I swear my research capability reduced while playing without anything changing(that I know of).

[Shark7]

Your research cap by research type is based on your population. You do still get bonuses based on characters and locations though.

[FerretStyle]

The research cap encompasses all of your research.

Not sure what affects the cap... I'd be interested in knowing that, as well.

[feelotraveller]

The cap is of all 3 fields totalled.  So you can emphasise one field at the expensive of others.  If you have more lab research than your cap it is allocated proportionally.
 
The cap is derived from population and development (i.e. strategic value) summed over your empire.  It is a asymptotic cap, meaning it increases more slowly as you get more powerful.  The ultra-genius trait adds 20% to the total.
 
What you may be seeing with the decrease is the dissipating of the research bonus you get for certain story events in Shadows?  Look on your colony screen amongst the information for happiness/development influences to see if this might be the case.  Otherwise it could be anything that sets back development or population, such as disasters or fighting on the planet.