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The incidence of strong MgII systems in gamma-ray burst (GRB) spectra is afew times higher than in quasar (QSO) spectra. We investigate several possibleexplanations for this effect, including: dust obscuration bias, clustering ofthe absorbers, different beam sizes of the sources, multiband magnificationbias of GRBs, association of the absorbers with the GRB event or thecircumburst environment. We find that: i) the incidence rate of MgII systems inQSO spectra could be underestimated by a factor 1.3-2 due to dust obscuration;ii) the equivalent-width distribution of the MgII absorbers along GRBs isconsistent with that observed along QSOs thus suggesting that the absorbers aremore extended than the beam sizes of the sources; iii) on average, GRBafterglows showing more than one MgII system are a factor of 1.7 brighter thanthe others, suggesting a lensing origin of the observed discrepancy; iv)gravitational lensing (in different forms, from galaxy lensing to microlensing)can bias high the counts of MgII systems along GRBs if the luminosity functionsof the prompt gamma-ray emission and of the optical afterglows have a meanfaint-end slope approaching -5/3 -- -2; v) some of the absorbers can beassociated with the circumburst environment or produced by supernova remnantsunrelated to the GRB event itself but lying in the same star-forming region.With the possible exception of magnification bias, it is unlikely that one ofthese effects on its own can fully account for the observed counts. However,the combined action of some of them can substantially reduce the statisticalsignificance of the discrepancy.Comment: 8 pages, 3 figures, ApJ in press, comments welcom

Strong MgiiSystems in Quasar and Gamma‐Ray Burst Spectra