Massive galaxies at z>6 represent the rarest peaks in the density field. When searches attempt to detect these peaks as Lyman-break galaxies (LBGs), the shape of the photometric selection function gives the survey volume a non-negligible width in redshift-space. Because these LBGs are on the tail of the mass function, their abundance drops exponentially with redshift. The LBGs seen toward the near end of the survey are exponentially more abundant than those at the far end or even in the middle of the survey. This effect of viewing the survey volume along the light-cone rather than at a single, central redshift changes the abundance and clustering properties one expects to measure. In particular, the observed mass function in z- and J-dropout surveys is shallower than the 'true' function measured at a single redshift. Furthermore, because the clustering of such rare peaks is so large, finding one often implies the existence of others nearby, despite their rarity. It is therefore possible to verify the high-redshift identity of a galaxy (prior to an unambiguous spectral identification) from the clustering of other galaxies around it.
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