Rule of writing, using periods in text messages is 'less sincere'

By having a mental list of ideas for conversation topics.
Just picture this scenario: You are in the middle of a conversation by text messages with a friend. You make a simple and innocent enquiry as to when next you hope to see them. And their response comes. “Tomorrow!” Really, how would you feel? The surprise and the stress that such a response would elicit from you is better imagined than experienced. The reason for such a reaction from you would simply be because of the exclamation mark at the end of their response.
This singular punctuation mark appears to show that you and the recipient are having a strained relationship, whereas this might really not be the case.
In fact, the awkwardness of this scenario has been long suspected to be a major determinant in any smooth flowing text conversation. Now, science has gone a step further to reveal that their suspicions are actually founded.
It turns out that according to research studies conducted by Celia Klin of the Binghamton University, when you are texting and a period is placed at the conclusion of a sentence, the writer appears to be not sincere in their response. The findings of this study would only buttress the long held belief that texts which contain punctuation marks are not deemed to be grammatically correct by the recipients. Rather, the person receiving it would think you are simply being rude or prudish.

The fact is this: Klin has rightly pointed out that whenever you are sending a text message always realise that you are not in an English Language or Linguistics course where your grammatical abilities are being scrutinised and tested. A text message is simply a message between two familiar and friendly people. So the mode of messaging should be very relaxed and informal. This would go a long way to saving you from any embarrassments or terse responses as well.

 On the other hand

You should never end your text messages with a full stop.
"When speaking, people easily convey social and emotional information with eye gaze, facial expressions, tone of voice, pauses, and so on,"
But “Text messaging is one of the most frequently used computer-mediated communication (CMC) methods. The rapid pace of texting mimics face-to-face communication, leading to the question of whether the critical non-verbal aspects of conversation, such as tone, are expressed in CMC,”

more explanation:
Ending your texts with a full stop is truly monstrous. We all know this. Grammar be damned, it just doesn't look friendly.
Now a study has confirmed it.
Researchers studied the texting behavior of 126 undergraduates within exchanges framed  as either text messages and handwritten notes.
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i.e:
Even the most neutral texts can seem like stern responses when accompanied by a full stop. Send a “Fine.” instead of a “Fine!” when making plans and there’s a good chance the recipients of your message will wonder if you’re angry with them.
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