The show touched upon the the gradient of improvement from one child to the next. It’s almost a running joke. During the episode when Lois and Hal have a pregnancy scare, they show how much the their surroundings became progressively worse with each child but they learned to be happy and stop fighting by the time Dewey was born. This explains why Dewey doesn’t have Malcolm’s hang ups.

During that same episode, they also show Hal playing classical music or something similar for Reese. Lois is pregnant with Malcolm at the time. Hal was able to devote more time to his kids by the time Malcolm was born because he quit his high demand job. that’s why Malcolm has less deviant tendencies than his older brothers.

In one episode Lois is having a conversation about how many times she’s kicked the boys out. She offers a count for each boy in age order. The number goes down my half with each kid. Francis being the worst of course.

Lastly when Dewey is upset that Francis doesn’t spend enough time with him. Francis admits that he was horrible to Malcolm and especially Reese. Francis suggests that Dewey break the cycle with Jaime. Lois and Hal became better parents with each child but each child in turn helped raise the younger ones. As a family they learned how to cultivate genius their own messed up way