toddlers under the age of two and a half readily learn new words and actions from a responsive adult but pick up almost nothing from prerecorded instruction delivered on a screen—a phenomenon that researchers call the “video deficit.” Humans learn best from other (live) humans. Perhaps more surprising, people learn from teaching other people—often more than the pupils themselves absorb. Consider this finding: firstborn children have an IQ that is on average 2.3 points higher than that of their younger brothers and sisters. After disconfirming several potential explanations, such as better
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