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| Illustration by William B Hogan |
The human brain has been called “the
most complex three pounds in the universe.”[ 1]Indeed, this
characterization does not seem hyperbolic when we consider that we are born
with 100 billion neurons at birth,[ 2] and that an additional 250,000 to
500,000 new neurons are formed every minute in the first few months of an
infant’s life.[ 3]Further,it is not just the number of neurons,
but the number of synapses, or connections between
neurons, that is extraordinary. Synaptic connections become increasingly
complex in the first few years of life, and children have 1,000 trillion
connections by age three.[ 4]Early experiences are
critical in shaping this process.In
the brain, neural circuits that are used repeatedly tend to strengthen, whereas
those that are not used are dropped, or pruned. The most vigorous growth and
pruning of these connections occur in the first three to four years of life,[ 5]meaning that the
brain is most plastic, or able to make new connections, early in childhood.
For example, studies
in the late 1990s revealed that children who learn a second language early (prior
to age seven) show neural organization of the second language that is
remarkably similar to that of the first language. In contrast, among late
learners of a second language, the second language is in effect “stored
separately,”
[
6
]
which helps to account for poorer pronunciation
and grammar in late-second-language learners.
More recently, the
effects of early life experience have been applied to the study of the aging
brain. As we get older,
the function
of our nervous system declines. For example, older adults often have difficulty
understanding speech as well as their younger counterparts. This difference is
particularly salient in environments with substantial background noise, such as
cocktail parties. Researchers recently showed that several years of music
training early in life can offset this process of auditory decline.
[
7
]
Specifically, a moderate amount of
musical training in early childhoodis associated with faster neural
response to speech later in life, decades after the individual last picked up a
musical instrument. The research suggests that early experience with music
trains the brain to interact more dynamically with sound throughout a person’s
life.
While early exposure
to additional languages or music may lead to beneficial changes in brain
development, early adversity can likewise have important but detrimental effects
on the brain.
Early Adversity
Children under 18
years of age represent 23 percent of the population, but they comprise 34
percent of all people in poverty. More than one in five children in the United
States live in poverty, representing more than 16 million children.
[
8
]
Importantly, the
definition of povertyis strictly
based on family income and the number of adults and children in the home, with
no adjustment for geographic location. Thus, based on the most recent federal
guidelines, a family with two adults and two children is considered to live
below the poverty line if they earn less than $23,550 per year, regardless of
where they reside. The guidelines fail to take into account the fact that raising
a family in a city like New York or San Francisco is much more expensive than,
for instance, raising a family in rural South Dakota.
For that reason, many
researchers partly consider the role of multiple socioeconomic factors in
addition to income. Socioeconomic status, or SES, incorporates additional objective
measures such as parental education and occupation. Sometimes researchers also
consider subjective social status, which is an individual’s subjective rating
of his or her position in the social hierarchy.
Across these
different socioeconomic indices, researchers have described marked disparities
in a range of important cognitive and achievement measures for children, such
as IQ, literacy, achievement tests, and high school graduation rates.
[
9
]
Disparities inachievement tend to emerge early and then
widen throughout the early elementary school years. For example, by 10 years of
age, family SES is an excellent predictor of a child’s cognitive abilities:
children from higher-SES families tend to perform well above children from
lower-SES families—regardless of whether those children had high or low
cognitive abilities at age two.
[
10
]
Numerous factors
contribute to these SES gaps in cognitive development: nutrition, environmental
toxins, home learning environment, exposure to stress, and early schooling. Further,
these different pathways are often highly correlated, in that disadvantaged
families are more likely to be exposed to multiple risk factors than are
advantaged families. As such, researchers find it daunting to tease out the
mechanisms behind the SES gap in cognitive development and, in turn, to design
effective interventions.
Weighing Other Factors
One way to begin to
make sense of the tangled web of inter-correlated mechanisms leading to socioeconomic
disparities in cognitive development is to recognize that cognitive development
is itself a very broad construct—too broad to be realistically considered as a
single outcome. We are therefore better off trying to understand the links
between SES and specific aspects of
cognition.
The field of
cognitive neuroscience teaches us that different brain structures and circuits
support distinct kinds of cognitive skills. While classic academic milestones such
as school graduation can tell us broadly about global effects of socioeconomic disparities
on achievement, we know that achievement is actually a complex output of
multiple cognitive and socio-emotional systems, such as language, learning and
memory, and self-regulation. These distinct cognitive systems are supported by
different brain regions and networks. So, while classic measures of academic
achievement such as high school graduation must at some level reflect the function
of the brain, they are relatively uninformative when it comes to disruptions or
disturbances in specific cognitive and neural processes. By taking a cognitive neuroscience
approach, we may improve our efforts at providing targeted educational
interventions.
This was the approach that my colleagues and I have taken, beginning
when I was a graduate student with Martha Farah at the University of
Pennsylvania, in a series of studies over the last decade.
[
11-13
]
In these studies, we investigated
which core cognitive functions were most strongly related to SES. To do so, we
recruited children from socioeconomically diverse families and administered a
series of cognitive tests designed to tap in to the core systems of language,
executive function, visuospatial skills, and memory—systems that are supported
by relatively distinct brain circuits. Across studies, children ranged from
kindergarten through middle school age. At any one age, of course, some
children perform dramatically better than others. We set out to determine the
extent to which such disparities in performance could be explained by differences
in SES.
The answer, it turned
out, was “to a large extent.” In general, children from higher SES homes tended
to perform better on most cognitive skills than children from lower SES homes.
However, the disparities were not uniform. Across studies, we found the largest
SES disparities in language skills, with more modest differences in children’s
memory and executive-function abilities. For example, in one study, for each
standard deviation increase in SES (operationalized as a composite of parental
education, occupation, and income), language improved by more than half a
standard deviation, declarative memory skill increased by approximately
one-third of a standard deviation, and certain executive-function skills
increased by approximately one-quarter of a standard deviation.
[
11
]
Similarsocioeconomic gradients in these
skillshave been reported in
children in developing countries.
[
14
]
More recent work from
our lab has suggested that socioeconomic disparities in neurocognitive
development emerge very early, with large differences in language and memory development
evident before two years of age.
[
15
]
Building on the Findings
Scientists leading other
recent investigations of socioeconomic differences in brain structure and
function have considered more specific cognitive and neural outcomes, and it
has become possible to begin to tease apart the modifiable environmental
factors that mediate these links.
Since the greatest
socioeconomic disparities are present in language skills, let us turn first to
several findings concerning SES disparities in the function and structure of
language-supporting regions of the brain. Polish neuroscientist Przemyslaw Tomalski
and colleagues recently used electroencephalography (EEG) to examine SES
differences in infants’ brain function.
[
16
]
This technique is widely used by investigators
to examine how powerful a child’s brain waves are in different locations across
the scalp, thus providing some insight into the activity in different brain
regions. Their study found that by six months of age, parent occupation and
income were already associated with higher-power brain waves in frontal brain regions.
Critically, higher-power brain waves in these regions have been associated with
better language development at later ages.
[
17
,
18
]
Thus, it is possible
that at least one neural signature of growing up in socioeconomic disadvantage
may be detectable very early in infancy, well before behavioral measures of
discrepancies in cognitive processing may be evident.
In a recent study in
our lab, we examined brain volumes in a group of 60 socioeconomically diverse
children ranging from 5 to 17 years of age. We found that, as children get
older, higher SES children tend to dedicate relatively more neural real estate
to areas of the brain that support language development, in comparison to their
lower SES peers.
[
19
]
This suggested to us that something
about the experience of growing up in a higher SES environment likely leads to
a greater investment in language-related regions of the brain.
Indeed, this
something is almost certainly experience with language itself. It is well
established that children from disadvantaged homes tend to hear fewer words—an estimated
30 million fewer words by age three than their higher-SES counterparts, to be
precise.
[
20
]
Lower-SES mothers are also more likely
to speak to their children in a directive rather than conversational manner,
and to use less complex speech patterns and fewer gestures.[20-
22
]
It is likely that differences in maternal speech input
result in a cascade of effects that are directly relevant for the development
of a child’s language-supporting cortex during infancy.
[
21
]
Much as greater exposure to music may
increase an individual’s perception of speech years later, greater social
engagement with interactive adults may lead children to have improved abilities
to perceive and discriminate among speech sounds.
[
22
]
Thus, one mechanistic pathway would
suggest that socioeconomic disparities result in large differences in quality
and quantity of linguistic exposure, which in turn lead to differences in the
development of language-supporting brain regions—and, finally, to the
often-reported SES disparities in children’s language skills.
[
19
]
The Role of the Hippocampus
As described above,
SES disparities in children’s learning and memory abilities have been also reported,
independent of disparities in language. The
hippocampus is one brain structure that is critical for memory development, and
a number of recent studies have indicated that SES factors are associated with
hippocampal size in both children
[
19
,
23
,
24
]
and adults across the life span.
[
25
,
26
]
Research in
both animals and humans suggests that the experiences of
stress and neglectful or abusive
parents have direct effects on the
development of the hippocampus.
[
27-30
]
While family stress is certainly not limited
to lower SES families, it is often disproportionately felt in more
disadvantaged homes. Thus, a second
pathway would suggest that SES differences in exposure to stress may operateon the hippocampus to mediate
previously described SES disparities in declarative memory processes.
[
19
]
Supporting this notion, Joan Luby of
Washington University-St. Louis and colleagues recently found that more hostile
parenting relationships and family stress accounted for links between income
and hippocampal size.
Finally, s
ocioeconomic
disadvantage is associated with a decreased ability to regulate cognition
[
11-13
,
31
]
and emotions,
[
32-34
]
a critical aspect of school readiness that predicts grades
and achievement-test scores from elementary through high school. Recent work
from a number of laboratories has demonstrated SES disparities in the
neuroanatomic structure and function of the prefrontal and limbic cortical
regions that support these skills.
[
19
,
35-40
]
Again, chronic stress has been associated with alterations in the development
of this circuitry.
[
29
,
41
]
Thus,
a third pathway would suggest that SES differences in exposure to stress may also
operateon prefrontal cortex and
limbic circuitry, thus mediating previously described SES disparities in self-regulation.
[
19
]
For
example, NYU developmental psychologist Clancy Blair and colleagues
[
42
]
reported that the link between positive
parenting behaviors and children’s executive function was partially mediated
through the stress hormone cortisol, and Nim Tottenham of Columbia University
recently showed that early adversity in the form of maternal deprivation leads
to premature adult-like connections between prefrontal and limbic regions.
[
41
]
Thus, mounting evidence
suggests that socioeconomic factors—parental education or family income—may lead
to differences in the home-language environment or exposure to family stress,
which in turn have cascading effects on the development of brain systems that
support critical neurocognitive functions such as language, memory, and
self-regulation. And yet we still do not know the level at which it is most
efficacious to intervene.
Closing the Gap
Are our efforts best
directed at improving disadvantaged children’s educational experiences, with a focus
on language, memory, and self-regulation skills? School-based interventions are
certainly the most prevalent form of early-childhood intervention, and many,
such as the Chicago School Readiness Project
[
43
]
and Boston’s pre-K program,
[
44
]
have shown promising
gains in both preacademic and self-regulatory skills for disadvantaged
children. And yet, while these programs can be effective, they are unlikely to
be sufficient: Given the size of the SES gap by the time children enter school,
preschool interventions alone are unlikely to bridge the gap fully.
[
45
]
Some small, intensive early childhood
programs such as Perry Preschool or Abecedarian have been shown to result in
substantial long-term benefits on cognitive development and achievement, and
even physical health as children enter adulthood.
[
46
,
47
]
However, the pragmatics of scaling up
such programs to the larger population while maintaining high quality is a frequently
cited concern.
Young children spend
the vast majority of their time with their parents and other caretakers, and so
perhaps we should we be focusing on parents’ behaviors. Highly educated parents
invest far more time playing with, talking to, and teaching their children, and
parenting style has been cited as the single most important factor in
explaining the SES gap in cognitive development.
[
48
]
And so, perhaps targeting
parenting would be the most effective avenue of intervention. Small-scale
interventional efforts to teach disadvantaged parents about the benefits of
speaking early, often, and richly to their children are producing promising
results.
[
49
]
However, while some larger-scale
parenting interventions such as the Nurse-Family Partnership program have led
to moderate improvements in children’s cognitive and behavioral outcomes,
[
50
]
many have a
mixed record of success,
[
51
]
often due to difficulties with attrition and low participation.
Overcoming obstacles to scaling up such interventions will require researchers
and policy makers to carefully consider parental motivations and beliefs.
[
45
]
Finally, let us
consider interventions that operate at the most distal level—that of SES
itself. Correlational evidence suggests that, for disadvantaged families, a
$4,000 increase in family earnings in the first two years of a child’s life
leads to remarkable differences in that child’s adult circumstances, including a
19 percent increase in adult earnings, a marked increase in hours spent in the
workforce, and even some evidence of improved physical health in adulthood.
[
52
,
53
]
While family income alone is unlikely to be the most important
factor in setting young children along an achievement trajectory, it may well
be the most malleable factor from a policy perspective. Thus, based on the
evidence described above, many leading social scientists and neuroscientists
believe that policies that reduce family poverty would have meaningful effects
on early caregiving and reductions in family stress, ultimately improving
children’s brain functioning and promoting the cognitive and socio-emotional
development that is so critical for children to succeed and to lead healthy,
productive lives.
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