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Laughter is a form of non-verbal vocalization common to humans and other animals. However, in other animals laughter is restricted to a response to tickling whereas humans also laugh in response to different emotional states. Thus, laughter in humans has likely evolved as a complex form of social interaction.
Laughter is a rhythmic, audible contraction of the diaphragm and other parts of the respiratory system that occurs in humans and some other species of primate. In other apes, like chimpanzees and orangutans, laughter takes place when physically induced and is different in terms of structure and frequency from human laughter. Chimpanzee laughing is breathy and lacks vowel sounds and harmonic structure. Chimpanzee laughter consists of only one sound, produced via inward and outward breath, whereas human laughing is modulated to produce sound.
In humans, laughing is a form of positive expression that takes place mostly in social contexts. It is involved in social bonding, affection and emotional regulation. Human laughter has a harmonic structure with a fundamental frequency and vowel like sounds (ha). Human laughter also has more vibration regimes and egressive airflow.
Human specific laughter may have evolved as a result of bipedalism. In bipedality, the thorax is freed of its supportive role in quadrapedal locomotion, and as such, breathing is uncoupled from running. As a result, humans have better breath control, giving rise to speech and laughter specific to humans.
Selection for laughter in humans may have evolved in response to increased group size. As humans expanded their habitat, this expression of positive emotion could have been beneficial.
Laughter occurs in other primates. However, it is in response to playful activities like tickling and wrestling. The acoustic structure of human and non-human primate response to tickling is similar. Thus, it is likely that human laughter evolved from non-human primates but diverged to attain different functions under different circumstances.
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