From “Psychology Today”:
With neuroscience, we can confirm what our ancestors took for granted—that letting babies get distressed is a practice that can damage children and their relational capacities in many ways for the long term. We know now that leaving babies to cry is a good way to make a less intelligent, less healthy but more anxious, uncooperative and alienated person who can pass the same or worse traits on to the next generation.
You should read the whole article. You can Google it. It’s called “Dangers of ‘Crying It Out.'”