The 10 mysteries of human behaviour
Scientists have split the atom, put men on the moon and discovered the DNA of which we are made, but there are 10 key mysteries of human behaviour which they have failed to fully explain.
The New Scientist magazine compiled a list of the everyday aspects of life, including the reasons behind kissing, blushing and even picking your nose.
An editorial in the publication said: "There is nothing more fascinating to most of us than ourselves. So it is hardly surprising that we have expended large amounts of effort trying to get to the bottom of what it means to be human.
What is surprising is that there are so many traits that remain enigmatic. These range from the sublime, such as art, dreaming and altruism, to the ridiculous, think pubic hair, blushing and nose-picking.
They may seem quirky but the best explanations for them often have profound implications."
Here are some theories on why we do those things we do and some of the problems:
1 - Blushing: Charles Darwin struggled to explain why evolution made us turn red when we lie, which alerts others. However, some think it may help diffuse confrontation or foster intimacy by revealing weakness.
2 - Laughter: mood-improving endorphins are released when we laugh, which seems an obvious reason to do it but a 10-year study muddied the waters when it found more laughter is produced by banal comments than jokes.
3 - Kissing: the explanation for kissing is unlikely to be genetic as not all human societies do it. There are theories that it is associated with memories of breastfeeding and that ancient humans weaned their children by feeding them from their mouths, which reinforced the link between sharing saliva and pleasure.
4 - Dreaming: Sigmund Freud's theory of dreams expressing our subconscious desires have been generally discredited and it is recognised that they help us process emotions, but the reason why we see such strange visions has not been properly explained.
5 - Superstition: unusual but reassuring habits make no evolutionary sense; however, ancient humans would have benefited from not dismissing a lion's rustle in the grass as a gust of wind. Religion seems to tap into this impulse.
6 - Picking your nose: the unappealing but common habit of ingesting 'nasal detritus' offers almost no nutritional benefit, so why do a quarter of teenagers do it, on average four times a day? Some think it boosts the immune system.
7 - Adolescence: no other animal undergoes the stroppy, unpredictable teenage years. Some suggest it helps our large brain reorganise itself before adulthood or that it allows experimentation in behaviour before the responsibility of later years.
8 - Altruism: giving things away with no certain return is odd behaviour in evolutionary terms. It may help with group bonding or simply give pleasure.
9 - Art: painting, dance, sculpture and music could all be the human equivalent of a peacock's tail in showing what a good potential mate someone is. However, it could also be a tool for spreading knowledge or sharing experience.
10 - Body hair: fine hair on the body and thick hair on the genitals is the opposite of what occurs in primates, our close animal relatives. Suggested reasons for pubic hair include a role in radiating scent, providing warmth or even protecting from chafing.