An important ingredient in string theory is supersymmetry: the conjectured symmetry which pairs every fermion with a boson. For instance, the spin one photon would have a spin-1/2 partner called the "photino." Supersymmetric extensions of the standard model of particle physics are actively studied because of their theoretical advantages. For example, they keep higher order terms in perturbation theory from making the Higgs mass blow up (a problem in the standard model), and they provide a good explanation for the dark matter of the universe.
If one requires the Lagrangian of a theory to be locally invariant under supersymmetry transformations, one obtains a theory called supergravity. In the same way that quantum electrodynamics derives the existence of the photon by imposing a local gauge symmetry, the requirement in supergravity of local supersymmetry implies the existence of a spin-3/2 particle called the gravitino. Then supersymmetry requires the existence of its spin-2 supersymmetric partner, the graviton, which is the quantum particle of gravity.