with respect to our mass-selected sample.

The median UV slope $\beta$ is $\langle \beta \rangle=-0.36$, indicating a relatively flat spectrum in $F_{\lambda}$. The distribution of $\beta$, ranging from $\beta=-0.95$ to $\beta=1.10$, is broadly consistent with the distribution of massive galaxies at $2<z<3$ from @vandokkum06. As shown in the bottom panel of Figure \[fig-histos\], the distribution of $\beta$ is instead very different from the distribution seen for $H-K>0.9$ galaxies at $z\sim3.7$, which shows a peak at $\beta \sim -2$ [@brammer07]. The observed distribution of $\beta$ for the mass-selected sample at $3.0 \leq z < 4.0$ is also very different from the distributions seen for UV-selected galaxies at $z\sim2.5$ and $z\sim4$, which peak at $\beta \sim -1.6$ and $\beta \sim -1.8$, respectively (e.g., @adelberger00; @ouchi04; @hathi08; @bouwens09).

The intrinsically different rest-frame UV properties of the mass-selected sample at $3.0 \leq z < 4.0$ studied in this work and the typical UV-selected galaxies at these redshifts (i.e., $U$- and $B$-dropout galaxies) is also clear from Figure \[fig-dropouts\], which shows the location of the massive galaxies at $3.0 \leq z < 4.0$ in the $U_{\rm n}GR$ and $B_{\rm 435}V_{\rm 606}z_{\rm 850}$ diagrams commonly used to select $U$-dropout galaxies (i.e., LBGs; @steidel03) and $B$-dropout galaxies 