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We combine old and new quantum field theoretic arguments to show that anytheory of stable or metastable higher spin particles can be coupled to gravityonly when the gravity sector has a stringy structure. Metastable higher spinparticles, free or interacting, cannot couple to gravity while preservingcausality unless there exist higher spin states in the gravitational sectormuch below the Planck scale $M_{\rm pl}$. We obtain an upper bound on the mass$\Lambda_{\rm gr}$ of the lightest higher spin particle in the gravity sectorin terms of quantities in the non-gravitational sector. We invoke the CKSZuniqueness theorem to argue that any weakly coupled UV completion of such atheory must have a gravity sector containing infinite towers of asymptoticallyparallel, equispaced, and linear Regge trajectories. Consequently,gravitational four-point scattering amplitudes must coincide with the closedstring four-point amplitude for $s,t\gg1$, identifying $\Lambda_{\rm gr}$ asthe string scale. Our bound also implies that all metastable higher spinparticles in 4d with masses $m\ll \Lambda_{\rm gr}$ must satisfy a weak gravitycondition.

Closed Strings and Weak Gravity from Higher-Spin Causality