Nancy Bishop, in her doctor's thesis on The Barberini Gospels for the University of Iowa, 2004, page 55, interpreted the beard-puller as a Jew. "The combination of the beard-pulling , nakedness , squatting and the gesture toward the circumcised penis all support this identification. His placement within a group of active, writhing serpents could have been intended to represent hell, a vision fleshed out in early vision literature such as the Visio Sancti Pauli».
However, the penis does not look circumcised to my practised eye, but sub-erect with retracted foreskin, as on many male exhibitionists, few of which are circumcised. Nor does the man have the humiliating headgear that Jews were usually forced to wear and depicted as wearing. He is shown on one side of a page of Canon Tables (correlations in the Gospels) which would be an odd place for a Jew to be. The beard is the forked beard standard in Romanesque art. It seems to me evident that this figure is a warning to monkish readers of the terrible and eternal consequences of sexual acts within the monastery.