It occurs to me that I have direct experience and knowledge of a situation in which a parenting style has created not one, but two manipulative children, and I was able to draw a straight line between cause and effect. I didn't think about them at first because they're not nearly teens yet. When I was last interacting with them regularly, they were 8 and 10.
The cause is lack of consistency, and standing of ground by the parents. The kids have learned that "no" doesn't mean "no," it means, "keep trying." They've gotten very used to getting their own way at home. The older boy mostly relies on a brute force hacking method... keep bothering the parents relentlessly, and eventually they placate him to get some peace. The girl uses that method, too, but she also uses more subtle means.
The kids then apply these same principles to their other relationships, with disastrous results. The older boy is a social outcast among his peers, with zero friends. We've witnessed the girl destroying more than one relationship she highly valued... including the one with our DD.
The boy has been identified as gifted, and we see a lot of the traits in him. The girl has not, and we don't see it in her.
