How Does a Proof-of-Stake (PoS) System Deter a 51% Attack?

A PoS system deters a 51% attack by making it prohibitively expensive to acquire 51% of the total staked cryptocurrency (the native token). If an attacker successfully gains 51% of the stake and attempts to create a malicious chain, the protocol's "slashing" mechanism will detect the double-signing and automatically destroy (slash) the attacker's staked funds.

This guaranteed, permanent loss of capital makes the attack economically irrational, as the attacker's investment would be immediately and significantly devalued.