( CNN ) -- People have always known that inequality is divisive and socially corrosive . What is surprising , now that we have the data to compare societies , is how clear the effects of inequality are . A wide range of social problems are worse in societies with bigger income differences between rich and poor . These include physical and mental illness , violence , low math and literacy scores among young people , lower levels of trust and weaker community life , poorer child well-being , more drug abuse , lower social mobility and higher rates of imprisonment and teenage births . The differences in performance of more and less equal societies is often enormous : Most of these problems are between twice and ten times as common in countries like the United States , Britain and Portugal , which have large income differences compared to countries with smaller income differences like the Nordic countries or Japan . For example , taking high , medium and low inequality countries , the homicide rate in the United States in 2009 was 50 per million population compared with 18 in Canada and 5 in Japan . The police , prisons and public services needed to defend ourselves against these problems are expensive and often not very effective . But the underlying causal processes are fairly clear . The problems that get worse when there is more inequality are all problems that become more common lower down the social ladder within each society . TED.com : What explains trust and morality . Greater income inequality seems to amplify and intensify the effects of social status differentiation -- bigger material differences creating bigger social distances . So the most common trigger to violence seems to be people feeling disrespected and looked down on . Although social class imprints its effects on us from earliest childhood onward , greater inequality makes these effects more marked . But inequality does not harm the poor alone . The effects are so large because almost everyone is affected . The benefits of greater inequality are biggest at the bottom of society , but a number of studies suggest that a large majority -- perhaps 90 % or 95 % -- of the population benefits from greater equality . We can not say what happens to the superrich because they are a fraction of 1 % of the population and we do not have separate data on their health , violence or drug use . Because position in the hierarchy has always been important to well-being , we have an inherited sensitivity to social status that works rather like ranking systems among some monkeys . However , human beings have lived in every kind of society from the most egalitarian ( such as the hunting and gathering societies of human prehistory ) to the most tyrannical dictatorships . Where there is more equality we use more cooperative social strategies , but where there is more inequality , people feel they have to fend for themselves and competition for status becomes more important . TED.com : How we read each other 's minds . Crucially important is the quality of social relationships . Because members of the same species have the same needs , they can , all too easily , be each other 's worst rivals -- fighting for food , nesting sites , territories , sexual partners and so on . But human beings , as well as having the potential to be each other 's most feared rivals and competitors , also have the opposite potential : We can be each other 's best sources of cooperation , assistance , help , learning and love . Depending on our social relationships , other people can be the best -- or the worst . Of paramount importance in our social development was to avoid conflict and competition for basic necessities . That is why we eat together . It is also what the religious symbolism of communion is about . Whether society has great inequality and a strong status hierarchy , whether there is a strong sense of superiority and inferiority , tells us whether we are in the same boat together and depend on cooperation and reciprocity , or whether we have to fend for ourselves in a dog-eat-dog society . What matters is not simply adults ' recognition of inequality and social status ; it is also a matter of how the parental experience of adversity is passed on to children to affect their early development . The sensitive period in early childhood that shapes development exists in many different species . Its function is to enable the young to adapt to the kind of environment they will have to deal with . Among human beings , that is primarily a matter of adapting to the social environment : . Are you growing up in a world where you will have to fight for what you can get , watch your back and learn not to trust others because we are all rivals , or are you growing up in a world where you will depend on cooperation , reciprocity and mutuality ? * * * . If you want a fuller picture of how we are affected by inequality or would like to use our slides or read answers to various `` frequently asked questions , '' please go to www.equalitytrust.org.uk . A more detailed but readable presentation of the evidence can be found in our book , `` The Spirit Level , '' which I wrote with Kate Pickett , who is , like me , an epidemiologist . ( Note : in England a `` spirit level '' is simply the tool used by carpenters and builders that most American 's call a `` level . '' ) The opinions expressed in this commentary are solely those of Richard Wilkinson .
Richard Wilkinson : Evidence confirms that inequality causes social problems . Social ills in U.S. , U.K. can be up to 10 times greater than in more equal countries . When there 's more inequality , people become desperate for higher social status , he says . Wilkinson : Children trained either for world of constant struggle or for cooperation .