  The correlation between radio spectral index and redshift has long been used
to identify high redshift radio galaxies, but its cause is unknown. Traditional
explanations invoke either $(i)$ intrinsic relations between spectral index and
power, $(ii)$ environmental differences at high redshift, or $(iii)$ higher
inverse Compton losses due to the increased photon energy density of the cosmic
microwave background. In this paper we investigate whether the increased
inverse Compton losses alone can cause the observed spectral index - redshift
correlation by using spectral modelling of nearby radio galaxies to simulate
high redshift equivalents. We then apply selection effects and directly compare
the simulated radio galaxy sample with an observed sample with sufficient
redshift coverage. We find excellent agreement between the two, implying that
inverse Compton losses and selection effects alone can largely reproduce the
observed spectral index - redshift correlation.
