Quantum Probability assignment limited by relativistic causality

The quantum nonlocality is limited by relativistic causality, however, the reason is not fully understood yet. The relativistic causality condition on nonlocal correlations has been usually accepted as a prohibition of faster-than-light signaling, called no-signaling condition. We propose another causality condition from the observation that space-like separate events should have no causal relationship. It is proved that the new condition is stronger than no-signaling condition for a pair of binary devices. We derive the standard probability assignment rule, so-called Born rule, on quantum measurement, which determines the degree of quantum nonlocality, by using relativistic causality constraint. This shows how the causality limits the upper bound of quantum nonlocality through quantum probability assignment.