system
A chat on subreddit /r/NoStupidQuestions
human
The concept is an admission against interests.  

People will lie to get out of trouble, but almost nobody lies with the intention of getting in more trouble.   So a statement that yes I did (a crime) is considered more trustworthy by the court.
gpt
And technically simply uttering the words “I did it” is not conclusive proof of a crime. You need more context than that.