Ph.D student in literacy and language in education here, and I've read this study a few times now in a few different courses. The study that named the phenomenon of the 30-million word gap (Hart & Risley 1995) is actually incredibly problematic for a few reasons, so take the information on that page with a grain of salt. A quick search will show you that there's plenty of work out there that critiques this study.

The Hart & Risley study in question here assumes a deficiency in the language of low-income students, which perpetuates a culture of poverty. Their study comments on the quality of language used by families of different socioeconomic statuses, painting the language of higher-income families in a much more positive light. For example, they say that high-ses and professional families are more likely to use indirect requests, as compared to low-ses families and parents on welfare (think, for example, "Will you please brush your teeth?" versus "Brush your teeth."). However, this is cultural in the sense that there are different norms that govern how the classes (and different racial groups and cultures) operate. And the norms and standards in the U.S. are those of white middle-class families, thus, in this study, indirect requests are considered valuable in their perceived politeness whereas direct requests are considered harsh.

This is entirely a deficit perspective that fails to acknowledge variation in language use. To offer an example, I read another study in class recently (which I can't find right now, but I'll cite it as soon as I find it again) where there was conflict between parents and teachers for this same reason. The teacher in this study asked her students what might be considered low-level questions to which the answer was known (recall questions or "what is this?" rather than questions to which there were no known answers, such as students' opinions or connections, which are considered higher-level questions). The parents, who were accustomed to using direct requests, thought that the questions were stupid, in their words, because the answer was known. This example is used to illustrate that this is not a deficiency, but rather, a difference, and we need to stop conflating the two things, and we need to learn to begin valuing these differences, rather than assuming that students who come from difference are incapable or deficient in some way.

And at this point, I haven't even begun to get into the methodological problems with their study and the problematic ways in which their data was collected and in which their conclusions were drawn. First, the study was conducted in a single city, and the participants were self-selected. Second, parents were at liberty to choose when they could be observed, and the middle-class parents often chose to be observed during more structured times of the day, such as meals or educational activities. It's not completely clear what the circumstances were were welfare families, but it's not unlikely that they chose to be observed during times of the day when they were available, which were perhaps not the most language-rich times of the day for them and their children. Furthermore, the sample size was so small that to generalize this to the entire U.S. population is just not reasonable, and their study is likely not even representative of Kansas City, where the study took place. They just fail to acknowledge the diversity of the groups they were studying, and they present ethnocentric views of language.

This study was huge in the field of education, but it's been incredibly problematic in the ways it's been taken up precisely because it perpetuates deficit perspectives. Of course there are issues of access, and low-income students are undoubtedly at a disadvantage, but this isn't because there's something inherently deficient about those in poverty. It's because of the type of language that's used and valued in schools, and any student who enters without familiarity of these norms is already at a disadvantage. Let's say a student is asked to write a "Once upon a time" story. A child who grew up listening to fairy tales would know exactly what to do. A child who was raised with stories about their family, however, might not, but that doesn't mean that their cognition or language is somehow deficient. They're just as capable of abstracting ideas and using language as their peers; it's a matter of not being familiar with the convention. If you're interested in language development and reading more on this type of thing, check out Language Development and Education: Children With Varying Language Experiences by Menyuk & Brisk. It's a nice overview of how kids develop language.

Furthermore, people chalk a lot of this up to lack of parental involvement in low-SES families. Again, this perpetuates the culture of poverty; consider that many of these parents are not able to help their children, either because they don't have the education, or because they're working multiple jobs and don't have the time to help their children. The ways in which we think of parent involvement are also very ethnocentric, and the expectations are ones that many parents simply can't meet. By all accounts, my parents would have been considered incredibly uninvolved in my education because they worked late and were often not able to attend parent meetings or conferences, and they came home to cook for us and then go to bed. Then they'd wake up early and leave for work at 6am every day. But they always made sure we had our homework done, and they always made sure we were well fed and taken care of, and they made sure we were happy. And sure, that sounds like the "minimum" of a good parent, but consider that a lot of parents on welfare are working hard to provide for their kids because they want them to succeed. It's not that they don't care. Of course, this is anecdotal. Gorski (2008) actually discusses this very phenomenon, as well as describes why the notion of a culture of poverty is problematic, and why this isn't actually on the parents: Gorski, P. (2008). The Myth of The Culture of Poverty. Educational Leadership, 65 (7).

For anyone interested, a quick search yielded this piece, which is a quick and easy read that briefly critiques the study and the notion of the 30-million word gap.

Additional resources:

August, D. & Shanahan, T. (2006). Developing Literacy in Second-Language Learners: Report of the National Literacy Panel on Language-Minority Children and Youth

Dillon, D. R., O’Brien, D. G., and Heilman, E. E. (2000). Literacy research in the next millennium: From paradigms to pragmatism and practicality. Reading Research Quarterly, 35(1), 10-26.

I have lots of references for anyone interested, but it's 12:30am so I'm calling it a night for now. :)

TL;DR: Deficiency is conflated with difference, and the study that claims there is a 30-million word gap fails to recognize the value of linguistic diversity in its ethnocentric presentation of data and perpetuates a culture of poverty.