A writing task

As far I'm concerned, not all of the statements in the second exercise are myths.
I have firmly held beliefs that people use fewer gesture hands when telling a lie.
Indeed, when doing so, we are so focused on the words coming out of our mouth and on avoiding actions which could betray our lie that we pay attention to keeping control of our body and, therefore, use fewer hand gestures.

Concerning the other statements, I'm not sure about the one according to which avoiding eye contact would be a sure sign of deception. However, I can say that I make all the other signs of lying when thinking. (Do you mean, "...when I'm thinking about how to lie"? Or simply, "...when I'm thinking in general"? If you mean the second sentence, it would be more clear if you said "...I make all the other signs of lying when I'm simply trying to think.")
For this reason, I would say that the related statements are widely held beliefs which are simply not true.

A writing task

As far as I'm concerned, not all of the statements in the second exercise are myths.
I have firmly held beliefs that people use fewer hand gestures when telling a lie.
Indeed, when doing so, we are so focused on the words coming out of our mouths and on avoiding actions which could betray our lie that we pay attention to keeping control of our body and, therefore, use fewer hand gestures.

Concerning the other statements, I'm not sure about the one according to which avoiding eye contact would be a sure sign of deception. However, I can say that I make all the other signs of lies when thinking.
For this reason, I would say that the related statements are widely held beliefs which are simply not true.

This is really good :) keep it up !!