I dont think humans are by "nature " deceitful.
Thats said childhood can be a time of experimentation and we did test boundaries..hopefully learning by our mistakes.

Some pple think that by being deceitful they can gain some kind of an advantage over another for whatever reason. These pple dont seem to care who they hurt in the process.

I remember my younger sister wet  my shoes because she knew I would get into big trouble if my father thought I wet them myself. She told him what "I had done"..and watched me take the blame.
My dad did tend to not spare the rod.
I never forgot that.

I did tease my brother a lot when I knew he was not allowed to hurt me..I took advantage of that situation. He did get his revenge later on of course.

I deserved it.

I feel bad about that even now.

Sometimes can we also be honest..knowing full well we will get our own way anyway-knowing that what we say /do is going to be unpaleateable- believing the logic/mindset of the other is flawed for what ever reason and that they will just fall into line.?Honesty is not always what it seems on the surface

I think it depends on the person involved and what they/you have invested in them and vice versa.

hhhmmm,  mantra - I was playing pool with my "soon-to-be" brother in law at a pub.
It was a good guys day, his 'soon-to-be" wife " (my sister) rang up wanting to talk to him. i said he was not there.
They divorced a few years after, he was not a good catch.

But, was not a good thing for me to do.