Toy models for the influence of asymmetric drift and retrograde thick-disc stars on the vertical velocity and velocity dispersion gradients at |z| = 3h_zt. We use Eqs. (16) and (17) with v_c = 1, k_i = 1, σ_t = 0.2, assuming h_zT/h_zt = 1 − 4 and a fraction of retrograde stars f_r = 0 − 0.9. The crosses indicate h_zT/h_zt = 3. Circles stand for the N-body simulation fits of Fig. 8 below. Relatively small retrograde fractions of f_r = 0.1 − 0.2 cause lags of a few tens of per cent if the thin and the thick discs are well differenciated (i.e. h_zT/h_zt ≤ 3, as is the case for the galaxies in our sample).