  The observation of double parton collisions by CDF has provided the first
direct information on the structure of the proton in transverse space. The
actual quantity which has been measured is the `effective cross section'
$\sigma_{eff}$, which is related to the transverse size of the region where
hard interactions are localized. The actual value which has been measured is
sizably smaller than naively expected and it is an indication of important
correlation effects in the many-body parton distribution of the proton. We
discuss the problem pointing out a possible source of correlations in the
proton structure, which could have a significant effect on the value of
$\sigma_{eff}$.
