Predictions of scintillation for ground to space collimated Gaussian beams generated from a numerical wave optics simulation are compared with recent weak scintillation theory developed from the Rytov perturbation approach (L.C. Andrews, R.L. Phillips, P.T. Yu, Ap Opt 34 , p 7742-7751, 1995; J.D. Shelton, JOSA A 12 , p 2172-2181, 1995). Significant discrepancies are revealed for intermediate-sized beams, defined as beams whose initial diameters place the near ground turbulence in the transmitter near field and the remote space target in the transmitter far field. By adding wander tracking to the wave optics simulation, and by developing a separate analytic model of the beam wander scintillation mechanism, we show that the scintillation for intermediate-sized beams is dominated by turbulence-induced beam wander at the target, and that the results from the wave optics simulation are accurate. We conclude that the analytic theory"s treatment of beam wander is incomplete, leading to the output of incorrect predictions for the second moment of irradiance. The error is most severe at the target point on the transmitter"s optical axis.© (2004) COPYRIGHT SPIE--The International Society for Optical Engineering. Downloading of the abstract is permitted for personal use only.