不由自主 (bù yóu zì zhǔ) is a set phrase meaning 'can't help / involuntarily' — it describes a physical or bodily reaction you don't consciously control (e.g., shivering, laughing). 不禁 (bù jīn) is more about an immediate spontaneous emotional response, while 情不自禁 (qíng bú zì jīn) emphasizes being overcome by strong feelings. Unlike 不由得 (bù yóu de), which can also introduce an inevitable conclusion, 不由自主 is limited to involuntary actions.