It is demonstrated that in a two-stage scenario with elementary Poissonian emitters of particles (colour strings) arbitrarily distributed in their number and average multiplicities, the forward-backward correlations are completely determined by the final distribution of forward particles. The observed linear form of the correlations then necessarily requires this distribution to have a negative binomial form. For emitters with a negative binomial distribution in the produced particles distributed so as to give the final distribution also of the negative binomial form, the forward-backward correlations have an essentially non-linear form which disagrees with the experimental data. (C) 2000 Elsevier Science B.V.