Laughter, a positive, contagious and heartwarming expression can be scary sometimes. This phenomenon is due to our fears, which stem from a misalignment in what we see and what our expectations were.
If laughter sounds sinister, out of context or out of tune with what is happening (like a person laughing while seeing people dying in an accident) it sends a red flag to the mind, signalling that something is not right or that the person is not to be trusted.
Both laughter and fear are states of high arousal, common during intense emotions. A scream and a laugh are closely related and use the same emotional energy.
If a laughing person isn’t supposed to be laughing, or if the laugh is coming from a non-person, like a robot or a doll, there is a creepiness to it. Example: Alexa, the home virtual assistant from Amazon, started laughing out of the blue in many homes, terrifying families.