Trigger bias in large pT reactions

We explore how particle multiplicities in large transverse momentum reactions are affected by the way in which the detection apparatus is triggered. For example, we predict that the multiplicity of particles accompanying a large p_T hadron is increased when it is required that there be a second large p_T hadron in the opposite hemisphere. The mechanism responsible for this effect is shown also to make correlations between pairs of large p_T pions produced on opposite sides depend only weakly on their charges.We include a list of further experimental tests, feasible with available apparatus, that are suggested by the two-jet picture of large p_T reactions.