Electron-electron interactions in organic semiconductors split the lowest singlet and triplet states by the exchange energy, DeltaE(ST). Measurement of singlet and triplet emission spectra in a large number of conjugated polymers yield an almost constant DeltaE(ST) value close to 0.7 eV This is in contrast to the situation in molecules, where the exchange energy is found to depend on molecular size and to vary over a wide range. Quantum-chemical calculations are performed to address the origin of the constant exchange energy in phenylene-based conjugated polymers. The electron-hole separation in the lowest singlet and triplet excited states is found to be independent of the pi-conjugated backbone, and saturates for chains longer than a few repeating units, resulting in a constant exchange energy. In shorter conjugated, oligomers, confinement of the excitations destabilizes the singlet with respect to the triplet through exchange interactions and leads to a larger and size-dependent singlet-triplet energy separation.