The rich get richer and the poor get poorer. It's a trend that every free society says it wants to reduce or reverse, in favor of greater socioeconomic equality.
But how? Rich people tend to be smarter than poor people. Thus on balance they should learn more, earn more, and invest better. The rich and smart also tend to marry each other, which implies that their advantages-from better genes to better parenting and higher-quality education-may become more concentrated with each generation. They are in a virtuous circle. The poor, meanwhile, appear to be trapped in a vicious one.
Free public education seems like one way out of this trap. But although US children have had access to taxpayer-funded K-12 schooling for many decades, that hasn't stopped the country's socioeconomic divide from growing worse in recent years.
Could brain science come to the rescue? It seems an unlikely notion. Yet scientists in recent years have been uncovering potentially treatable factors, such as chronic stress, that likely help keep people at a low socioeconomic status (SES) by impairing their cognitive abilities.
The stress factor
Greater stress-from greater environmental noise, violence and abuse, poor parenting, and general disorder-has long been considered one of the unfortunate features of low-SES life. Stress is also a factor that can impair cognition. "We know that early life stress for rats results in lifelong diminished learning ability," says Martha J. Farah, a researcher at the University of Pennsylvania and a member of the Dana Alliance for Brain Initiatives.
Similar experiments on humans haven't been feasible, for ethical reasons. But Farah notes that there have been some "natural experiments" on humans, involving changes in ambient stressors such as noise. "When an airport was relocated in Germany, resulting in one town's schoolchildren being relieved of the airplane noise and another town's children becoming chronically exposed, test scores went up in the first town and down in the second," she says.
In another study, published in 2011, measures of cognitive ability among students in the US fell after local unemployment levels rose-most likely due to "reduced income and increased distress," concluded the authors. Joblessness also has been linked to faster cognitive decline in middle age.
The stress response involves the release of corticotropin-releasing factor (CRF), steroid hormones such as aldosterone and cortisol, adrenaline, and various other compounds. This response apparently evolved as an adaptation to improve our chances of survival amid adversity. But its activation-especially long-term activation-can bring about changes that weaken cognitive ability. Indeed, cortisol-like steroid drugs are known to have cognitive and psychiatric side-effects, and in rare cases can even cause a reversible "steroid dementia" syndrome.
Many brain regions contain neurons with receptors for stress response compounds, and thus in principle can be altered by stress. But the cognitive impact of stress seems to fall hardest on a few specific regions.
One is the hippocampus, which mediates the storage and retrieval of new declarative and spatial memories. Animal studies have found that stress or the application of stress hormones shrinks the dendrite "trees" through which hippocampal neurons receive inputs, dials down the production of new hippocampal neurons (neurogenesis), and on the whole shrinks this brain region.
The "executive" regions of the prefrontal cortex, and the emotion-mediating amygdala, also seem quite sensitive to stress. Stress evidently weakens the prefrontal regions that normally regulate the amygdala, allowing the latter to become more active and sensitive to emotional stimuli: The brain essentially shifts from a slower, more thoughtful mode to a faster, more impulsive and instinctive one. If the stress abates, cognitive ability may soon return to normal. But a very long-term stress during childhood, when prefrontal regions are still developing, might leave key cognitive functions such as impulse control, attentional capacity, and working memory permanently impaired. A brain-imaging study published late last year suggested as much: In a sample of 24-year olds, those who had lived in poverty and under long-term stress years before, as children, now showed less amygdala-regulating ability on average-but their impairment was unrelated to their current incomes.
Similarly, Cornell University researchers found in 2009 that a low-SES childhood was associated with a lower working memory capacity in young adulthood, and found evidence that the linkage was "mediated by elevated levels of chronic stress during childhood:" More stress led to even lower capacity. In 2011, researchers at Pennsylvania State University reported that a higher level of the stress hormone cortisol in the saliva of low-SES babies aged 7-24 months predicted worse executive functioning and IQ when the same children reached the age of three years. Their analysis suggested that traditionally recognized factors such as poor parenting affect a child's later cognitive abilities in part by raising his or her cortisol levels.
In 2012, Columbia University researcher Kimberly G. Noble and colleagues reported on an MRI study of children in which the researchers linked smaller hippocampal volume to lower parental income, and larger amygdala volume to lower parental education-both changes being consistent with the idea that low-SES situations tend to involve stress-induced cognitive impairment. And late last year, Farah and her colleagues reported that the MRI-measured thickness of two prefrontal regions in children was significantly determined by the mother's education level, a common marker of SES: More maternal education meant thicker prefrontal structures. "The two [mechanisms] that seem most likely are higher levels of chronic stress and lower levels of cognitive stimulation in the childhood experience of low-SES individuals," Farah says.
How much does maternal stress affect children when they are still in the womb? In which phases of development is the brain most sensitive to stress? And can the cognitive impact of chronic stress throughout childhood ever be reversed? These are among the questions that still need to be answered. Once they are, says Farah, "we'll be in a better position to think about interventions to promote neurocognitive flourishing in poor children."
Noble is finishing up a long-running study of SES and cognitive ability that is designed to address such issues. "We're measuring brain structure but we're also looking at various aspects of the home environment such as the home language environment, and stressful conditions," she says. "We're hoping to get a better sense of what are the explanatory factors that mediate these links between SES and particular brain regions."
A bandwidth-reduction effect?
Scientists recently have described a cognition-impairing effect of low-SES lives that resembles the effect of acute stress but seems to occur independently-and perhaps even more immediately within the brain.
In an experiment reported in Science last September, researchers did cognitive tests on 101 visitors to a New Jersey shopping mall, having first prompted their subjects to worry about their finances with hypothetical scenarios involving car-repair costs. The two cognitive tests used were a standard IQ-type reasoning test and a test of mental impulse-inhibition ability, or "cognitive control." Particularly after being prompted with high-repair-cost scenarios, the relatively poor mall-goers performed significantly worse on both tests than the richer ones did.
For a follow-on experiment in a more realistic financial setting, the scientists conducted the same cognitive tests on 464 small-scale sugar-cane farmers in India, before and after a harvest. On average, the farmers tended to score significantly worse before the harvest, when they were relatively short of cash, and better afterwards, when they had been paid for their crops. The researchers used statistical methods to control for (i.e., to eliminate the effects of) various confounding factors-such as differences in available time, and feelings of stress-in order to isolate a feeling-poor vs. feeling-rich effect.
How could a feeling of relative poverty decrease cognitive ability so swiftly and directly? The researchers hypothesize that when someone worries, even unconsciously, about not having enough money, the worrying ends up consuming some of the person's limited cognitive capacity, or "bandwidth."
"We think that when people face scarcity on any dimension, and in this particular case on the money dimension, it tends to involuntarily capture their attention, and thus there is much less bandwidth available for other tasks," says Anandi Mani, a researcher at the University of Warwick (UK) who was the lead author of the study.
The results seem broadly consistent with previous findings that cognition is a limited resource, which may be depleted by use, at least temporarily. In experiments, people who are forced to exert self-control-a key aspect of executive functioning of day-to-day adult decision-making-show evidence of having less of it on later tasks. As some researchers have argued, poverty is a state so bandwidth intensive that it may deplete self-control particularly quickly.
Mani and her colleagues plan further experiments, for example to look at poverty's effect on short-term versus long-term decision-making, as well as on parenting decisions. But already her work and others' in this area suggests that governments take this limited-cognitive-resource model into account when making policies that affect the poor. The idea, she says, should be to "to try to reduce the mental load that is imposed upon them"-by making paperwork easier, for example, or by sending reminders.