Conversation runs out quickly when talking to a newborn. They don’t say anything back. They won’t groan when you tell them it’s going to rain, or smile when you tell a joke.

At the same time, those early weeks are shrouded in a cloud of exhaustion. My baby didn’t sleep when he was meant to, which meant I couldn’t either. It’s no wonder that conversation wasn’t exactly flowing.

It starts to feel easier when they become more responsive, but it still didn’t come naturally to me to “coo” in response to my baby’s gurgles, or speak in “baby-ese” with big, loud, slow vowel sounds. I would often look in awe as other, seemingly more parental types, would have whole conversations with my baby.

A few months in, as babies start to respond more with babbles and giggles, it becomes easier. But studies show that some parents still do not speak to their children much, and that this can have lasting negative consequences – consequences even visible in the brain.

It was in the mid-1990s that a worrying discovery was made about a stark difference in language achievement in children. Researchers Betty Hart and Todd Risley went into homes of families from different socioeconomic groups, spending an hour each month recording them over more than two years.

Analysing the data, they found that children from the poorest backgrounds heard one-third as many words per hour as those from higher income backgrounds. Scaling up, they proposed that by the time the children were four years old, there would be a 30-million word gap between children from poor backgrounds compared to those in wealthier, professional households.

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This study was far from ideal. It had a small sample size, and it’s not clear if the word gap is as large as the researchers first suggested. Other critics have since shown that low-income children hear many more words than Hart and Risley reported when factoring in language they overhear from conversations both inside and outside the home. Responding to these critics, another group highlighted that “young children do not profit from overheard speech about topics of interest to adults”.