A paper published in 2008 showed that more than half of the spin of the proton comes from the spin of its quarks, and that the missing spin is produced by the quarks' orbital angular momentum. This work used relativistic effects together with other quantum chromodynamic properties and explained how they boil down to an overall spatial angular momentum that is consistent with the experimental data. A 2013 paper showed how to calculate the gluon helicity contribution using lattice QCD.