According to a study conducted by Celia Klin and a bunch of researchers at Binghamton University, text messages with a period attached to the end are thought to be less sincere than texts with no punctuation at the end.
Basically, giving a reply such as "Sure." to someone who asks if you want to hang out implies that you're a bit of a jerk, thanks to that little full stop right there. Bizarrely enough, the same effect did not apply to handwritten notes. The human brain is strange.
For their study, the team recruited 126 Binghamton undergraduates to read a series of 16 messaging exchanges in which the response either did or did not include a sentence-final period.
Based on the responses to the study, texts ending with a full-stop were seen as being much more more insincere than their period-less counterparts.
Paradoxically, texts ending with an exclamation mark are actually seen as highly sincere. "When speaking face-to-face, people easily convey social and emotional information with eye gaze, facial expressions, tone of voice, pauses, and so on," Celia Klin said. "Since texters can't use these, they rely on what they have available to them — emoticons, deliberate misspellings that mimic speech sounds and, according to our data, punctuation."