Harlow, experiments on primate learning
Filed with tags of Child Development, Cognitive Psychology, Infant Development, Social Interaction, Social Isolation, Social Psychology, "Harlow’s Classic Studies Revealed the Importance of Maternal Contact" is an article dated June 20, 2018, and posted by the Association for Psychological Science (APS).

Other sources summarize the Harlow studies of the 1950s and 1960s as follows
- https://www.simplypsychology.org/harlow-monkey.html

In this century, studies at a National Primate Research Center (NPRC) build on Harlow's work, with focus on anxiety and depression following periods of social isolation during a period of youthful brain formation.
- https://wisconsinwatch.org/2014/07/university-of-wisconsin-to-reprise-controversial-monkey-studies/
- https://isthmus.com/news/cover-stor...evive-controversial-primate-experiments/
- https://nprc.org/

Hart & Risley, observations on human learning
Despite pushback and controversy of a different nature, the documented early observations of Hart & Risley similarly underscore the importance of supportive interaction during youthful brain formation.
- NPR, June 1, 2018, Let's Stop Talking About the '30 Million Word Gap' https://www.npr.org/sections/ed/2018/06/01/615188051/lets-stop-talking-about-the-30-million-word-gap
Looking past the title or headline, the article agrees with the existence of a word gap which may be addressed. It also suggests other supportive actions may be taken, and using different terminology than "word gap" which may sound negative, judgmental, and deficit-based. Additionally, the article presents agreement that direct conversational interaction with a young child provides greater benefit than "ambient" or "bystander" conversations which a child merely overhears.
Originally Posted by NPR article
... the sheer volume of conversation directed at children, not just spoken in their presence, is fundamental to language learning and later success in school. All the cultural variation in the world "doesn't negate the fact that when you look at the averages, there is a problem here."

And what's most important, says Hirsh-Pasek, is that interventions inspired by Hart and Risley are nudging parents in the right direction. "We have made changes and movement in kids, in whole communities."
- Education Next, June 6, 2019, Don't Dismiss the 30 Million Word Gap Quite So Fast https://www.educationnext.org/dont-dismiss-30-million-word-gap-quite-fast/

Bottom Line
Time and attention are valuable, and giving these to young children appears to be especially important.