In the United States, a 2004 analysis estimated that more than 90% of deaf or hard-of-hearing children have two hearing parents, who usually don't know sign language. The striking result: unlike spoken languages, sign languages are rarely passed from parent to child. They spread mostly through schools, classmates and the Deaf community. Many deaf children therefore meet their language late, which can delay their language development.
In Kuşköy, a mountain village in northern Turkey, people talk across valleys by whistling Turkish. A 2015 study showed that whistled Turkish draws on both hemispheres fairly evenly, whereas spoken Turkish relies mostly on the left. The physical form of the signal, it turns out, matters to the brain.
Speech has no gaps between words. In 1996, researchers played 8-month-old babies a continuous stream of made-up syllables for just two minutes. The infants picked out the 'words' by tracking which syllables tend to follow one another. That's probability calculation, done without a calculator or any visible effort.