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Are people getting dumber?

Nov. 13, 2012
Courtesy of Cell Press
and World Science staff

Pres­sure to be in­tel­li­gent is weaker for peo­ple to­day than it was for our hunting-and-gathering an­ces­tors—and hu­mans may be get­ting grad­u­ally dumber as a re­sult, a sci­ent­ist is pro­pos­ing.

The bi­ol­o­gist, Ger­ald Crab­tree of Stan­ford Uni­vers­ity in Cal­i­for­nia, pre­dicts that hu­mans are likely to find a tech­no­log­i­cal so­lu­tion to the prob­lem, if it really is one, be­fore it gets out of con­trol. So we need­n’t en­vi­sion a fu­ture in which plac­id hu­mans end­lessly watch “re­runs on tele­vi­sions they can no long­er build,” he wrote, out­lin­ing his hy­poth­e­sis in the jour­nal Trends in Ge­net­ics.

The rea­son the prob­lem is likely to arise in the first place, he ex­plained, is that in­tel­li­gence and be­hav­ior re­quire op­ti­mal func­tion­ing of many genes. This re­quires enor­mous ev­o­lu­tion­ary pres­sures to main­tain. 

When hu­mans were in a prim­i­tive state, he wrote, na­ture would have ruth­lessly killed off those who weren’t smart enough to, say, spear down a large, dan­ger­ous an­i­mal. This re­lent­less culling would have kept the hu­man spe­cies on high alert, and bright. There is pres­sure to be smart to­day, he ob­served, but it’s sel­dom a life or death mat­ter an­y­more.

Hu­man in­tel­lec­tu­al abil­i­ties “per­haps reached a peak 2,000–6,000 years ago,” he wrote.

Crab­tree ar­gues that the the in­tri­cate web of genes en­dow­ing us with brain pow­er is par­tic­u­larly sus­cep­ti­ble to muta­t­ions, which are no long­er be­ing weeded out by a ruth­less nat­u­ral en­vi­ron­ment. “The de­vel­op­ment of our in­tel­lec­tu­al abil­i­ties and the op­tim­iz­a­tion of thou­sands of in­tel­li­gence genes probably oc­curred in rel­a­tively non-verbal, dis­persed groups of peo­ples be­fore our an­ces­tors emerged from Africa,” he said. In this en­vi­ron­ment, he added, in­tel­li­gence was crit­i­cal for sur­viv­al.

From that point, he con­tin­ued, we probably be­gan to slowly lose ground. With the de­vel­op­ment of ag­ri­cul­ture, came ur­ban­iz­a­tion, which may have weak­ened the pow­er of nat­u­ral se­lec­tion—that is, en­vi­ron­men­tal pres­sure—to weed out muta­t­ions lead­ing to in­tel­lec­tu­al dis­abil­i­ties. 

Based on cal­cula­t­ions of the fre­quen­cy with which harm­ful muta­t­ions ap­pear in the hu­man ge­nome and the as­sump­tion that 2,000 to 5,000 genes are re­quired for in­tel­lec­tu­al abil­ity, Crab­tree es­ti­mates that with­in 3,000 years (a­bout 120 genera­t­ions) we have all sus­tained two or more muta­t­ions harm­ful to our in­tel­lec­tu­al or emo­tion­al sta­bil­ity. More­o­ver, re­cent find­ings from neu­ro­sci­ence sug­gest that genes in­volved in brain func­tion are un­iquely sus­cep­ti­ble to muta­t­ions, he added.

But not to wor­ry, he went on: the loss is slow, and judg­ing by so­ci­ety’s rap­id pa­ce of dis­cov­ery and ad­vance­ment, fu­ture tech­nolo­gies are bound to re­veal so­lu­tions to the prob­lem. “I think we will know each of the mil­lions of hu­man muta­t­ions that can com­pro­mise our in­tel­lec­tu­al func­tion and how each of these muta­t­ions in­ter­act with each oth­er and oth­er pro­cesses as well as en­vi­ron­men­tal in­flu­ences,” said Crab­tree. “At that time, we may be able to mag­ic­ally cor­rect any muta­t­ion that has oc­curred in all cells of any or­gan­ism at any de­vel­op­men­tal stage. Thus, the brut­ish pro­cess of nat­u­ral se­lec­tion will be un­nec­es­sary.”

Re­cent mul­ti­-decade im­prove­ments in popula­t­ion-wide IQ scores—which have stopped in any case—don’t nec­es­sarily point to a gen­er­al in­crease in in­tel­li­gence of our spe­cies, Crab­tree ar­gued. He said this phe­nom­e­non may have been attributa­ble to ad­vanc­es in pre­na­tal care, pre­school educa­t­ion and pol­lu­tion con­trol.


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Pressure to be intelligent is weaker for people today than it was for our hunting-and-gathering ancestors—and humans may be getting gradually dumber as a result, a scientist is proposing. The biologist, Gerald Crabtree of Stanford University in California, predicts that humans are likely to find a technological solution to the problem, if it really is one, before it gets out of control. We needn’t envision a future in which placid humans endlessly watch “reruns on televisions they can no longer build,” he wrote, outlining his hypothesis in the journal Trends in Genetics. The reason the problem is likely to arise in the first place, he he explained, is that intelligence and behavior require optimal functioning of many genes. This requires enormous evolutionary pressures to maintain. When humans were in a primitive state, he wrote, nature would have ruthlessly killed off those who weren’t smart enough to, say, spear down a large, dangerous animal. This relentless culling would have kept the human species on high alert, and bright. There is pressure to be smart today, he observed, but it’s seldom a life or death matter anymore. Human intellectual abilities “perhaps reached a peak 2,000–6,000 years ago,” he wrote. Crabtree argues that the the intricate web of genes endowing us with brain power is particularly susceptible to mutations, which are no longer being weeded out by a ruthless natural environment. “The development of our intellectual abilities and the optimization of thousands of intelligence genes probably occurred in relatively non-verbal, dispersed groups of peoples before our ancestors emerged from Africa,” he said. In this environment, he added, intelligence was critical for survival. From that point, he continued, we probably began to slowly lose ground. With the development of agriculture, came urbanization, which may have weakened the power of natural selection—that is, environmental pressure—to weed out mutations leading to intellectual disabilities. Based on calculations of the frequency with which harmful mutations appear in the human genome and the assumption that 2,000 to 5,000 genes are required for intellectual ability, Crabtree estimates that within 3,000 years (about 120 generations) we have all sustained two or more mutations harmful to our intellectual or emotional stability. Moreover, recent findings from neuroscience suggest that genes involved in brain function are uniquely susceptible to mutations, he added. But not to worry, he went on: the loss is slow, and judging by society’s rapid pace of discovery and advancement, future technologies are bound to reveal solutions to the problem. “I think we will know each of the millions of human mutations that can compromise our intellectual function and how each of these mutations interact with each other and other processes as well as environmental influences,” said Crabtree. “At that time, we may be able to magically correct any mutation that has occurred in all cells of any organism at any developmental stage. Thus, the brutish process of natural selection will be unnecessary.” Recent multi-decade improvements in population-wide IQ scores—which have stopped in any case—don’t necessarily point to a general increase in intelligence of our species, Crabtree argued. He said this phenomenon may have been attributable to advances in prenatal care, preschool education and pollution control.