A court or police officer could legally compel you to press your finger onto your smartphone to unlock it, but if your phone is locked with a passcode, no one can legally compel you to open it, says William J. Cook, an attorney and partner at law firm Reed Smith in Chicago, who specializes in information technology, privacy, and data security. Cook explains that the difference between a password and a biometric identifier is great under the law–you have a right not to reveal the contents of your mind, which includes things like a password, but your fingerprints are a part of who you are and you expose them to the public every day. This is why when a person gets arrested, he or she must consent to fingerprinted while retaining the right to remain silent. Thoughts are protected, biometric identifiers (fingerprints, face, hair) are not.

Source: Here’s Why Lawyers Suggest You Stop Using Your Finger to Unlock Your Phone