Compelling input is language input (speech, text) that is not only interesting but compelling. It’s a hypothesis in language acquisition put forth by the linguist Stephen Krashen.

Compelling means that the input is so interesting you forget that it is in another language. It means you are in a state of "flow". In flow, the concerns of everyday life and even the sense of self disappear - our sense of time is altered and nothing but the activity itself seems to matter. Flow occurs during reading when readers are "lost in the book" or in the "Reading Zone". Compelling input appears to eliminate the need for motivation, a conscious desire to improve. When you get compelling input, you acquire whether you are interested in improving or not. Stephen Krashen

Scientific proof

So far, there haven’t been any proper scientific studies to prove or disprove the hypotheses. However, there are anecdotal examples of it being effective.

Everyone can probably point to things that they learned because they were interested in it and they got lost in “it” - whatever that “it” happens to be.

The 30-million-word gap

The "30-million-word gap" refers to a notable finding from a study conducted by researchers Betty Hart and Todd R. Risley in the 1990s. This study, published in their book "Meaningful Differences in the Everyday Experience of Young American Children," highlighted a significant disparity in the number of words that children from different socioeconomic backgrounds were exposed to by the time they reached the age of three.

The study followed children from various families and recorded the number of words spoken to them from infancy through age three. The results showed that, on average, children from wealthier, professional families heard about 30 million more words than children from less affluent, welfare-receiving families. This massive difference in early language exposure was correlated with corresponding differences in vocabulary development, language skills, and later academic success.

The term "30-million-word gap" has since been used to underscore the importance of early language exposure and the role it plays in cognitive development and educational outcomes. It also highlights the need for interventions that can help bridge this gap, such as encouraging more verbal interaction between parents and children in all households, regardless of socioeconomic status.

According to a survey done in 2018, 93% of primary school teachers and 95% of secondary school teachers in the UK believe “a lack of time spent reading for pleasure is a root cause of the word gap”.