  We present evidence that isolated growing discs, subject to internal spiral
perturbations, thicken due to both heating \emph{and} radial migration. We show
this by demonstrating that the thickness and vertical velocity dispersions of
coeval stars depend on their age as well as the change in their radii. While
the disc thickens due to internal processes, we find that this induces only a
minor amount of flaring. We further demonstrate the consequences of such
thickening on the structural properties of stellar populations and find that
they qualitatively agree with recent studies of the Milky Way disc.
