Introduction
Laughter is a universal human behaviour, and every human is genetically predisposed to develop the ability to laugh. Laughter was accorded high evolutionary significance by Charles Darwin (1872), and it is suggested that a primitive precursor to human laughter has evolutionary origins in the common primate ancestor of humans at least 6.5 million years ago.1 Humour and laughter have played an important role in human culture since the beginning of recorded time. The positive effects of humour and laughter are referenced in The Bible, Book of Proverbs 17:22 (NIV): ‘A cheerful heart is good medicine, but a crushed spirit dries up the bones’. This indicates that people in the 10th century already understood the health benefits of humour and laughter.2
Fry was a pioneering investigator who, in the 1960s, coined the term gelotology: the study of laughter. His work provided mechanistic insights and evidence for the positive physiological impact of laughter and humour.3 Various benefits of laughter include alleviation of pain of ankylosing spondylitis,4 allergic skin reaction,5 and radiation dermatitis6; positive effects on the neuroendocrine-immune system7; endogenous opioid release8; and improvement of blood vessel function,9 cardiovascular diseases,10 depression,11 subjective health,12 cancer care management,13 and functional disability.14 The link between laughter and gene expression was also demonstrated using DNA microarray techniques: of 18 716 genes, eight were relatively upregulated and 15 were downregulated after a laughing episode.15 Five of the eight upregulated genes were associated with cell adhesion, apoptosis and cell cycle. The beneficial effects of laughter on diabetic nephropathy in terms of normalising the expression of the prorenin receptor gene are strongly suggested.16 Pain and cortisol levels could be decreased by laughter-inducing therapies, which show promise, especially for depression, although further research is needed to support this conclusion.17
Although the brain laughter circuit has been discussed in the field of neurophysiology, and the beneficial effects of laughter are abundantly reported, the physical function that is required as a premise for laughter remains unknown. In fact, from the epidemiological point of view, an obstacle to triggering laughter and the extent of its negative effects on laughter have not been studied. Newborn babies laugh spontaneously as early as 5–9 weeks.18 The number of situations that elicit laughter, which mainly include tactile, social playing, auditory, and/or visual stimulation, increases with age.19 In adults, most opportunities to laugh follow visual or auditory stimulation,20 which is processed in the dorsal upper pons.21 Thus, we hypothesised that there may be a connection between visual impairment and laughter. Visual impairment could negatively affect laughter, and there is a possibility that those with visual impairment have limited opportunities to laugh in daily life.
The prevalence of visual impairment approximately triples with the progression of each decade of life beyond the age of 40 years and, in developed countries, the prevalence of eye diseases and visual impairment has been increasing, which is driven by population ageing.22 In Japan, half of the population with visual impairment is older than 70 years, and the number of visually impaired persons is projected to rise in the future.23
Therefore, we examined the relationship between self-reported visual status and laughter. If visual impairment is shown to have a negative effect on laughter, this would suggest the importance of preventing and treating visual impairment and implementing social support programmes for visually impaired individuals.