Nature After Nurture?

Smart-babyPhoto Credit: Angie Hill

by Meghan Rosen

Last year, while doing our taxes, my husband and I were surprised to discover that we weren’t as poor as we thought we were. As lowly graduate students making a combined income of about $50,000 per year, I had assumed we were on the penny-pinching side of the national pay scale. But when I compared our income to the median income in the country, I found that we were sitting comfortably in the center. We had made it; we were officially smack-dab in the middle class. I thought it would feel different.

In the United States, nearly 25% of the population makes less than $25,000 per year. At this bottom level, a few households squeak by the poverty threshold, but just barely: in 2010 it’s estimated at just $22,314 for a family of four.

This year, 16 million children will be born into poverty (1 out of every 5 children born in the US). The lives of these children often follow a common stagnant storyline: poor nutrition, delayed mental and emotional development, academic deterioration, criminal activity, and frequently, early parenthood. As young parents, they are more likely to be unwed, more likely to drop out of high school, and more likely to stay impoverished. The cycle is vicious, and unrelenting. But is it possible to escape? How early is the influence of our environment engraved into the patterns of our development?

In 2003, a study from the University of Virginia showed that 7 year-old fraternal twins raised in families with low socioeconomic status had almost no variability in IQ. Why is this surprising? Twin-tigers Fraternal twins are as genetically dissimilar as any other pair of non-twin siblings—their IQs should have been different.

Unlike identical twins, which come from the same blend of a single sperm and a single egg’s DNA, and have matching sets of genes, fraternal twins are completely unique. Two eggs and two sperm form two separate embryos: two genetically distinct individuals that share only their time and space together in the womb. The height, build, athletic ability, and IQ of one fraternal twin can be as different from the second as any of their other brothers or sisters. In fact, differences between fraternal twins are not only common; they are expected.

Why then, were these differences not reflected in the mental abilities of the 7-year olds? Is it possible that variation in IQ doesn’t occur until later in childhood? Or did their low socioeconomic environments somehow mask their inherent genetic potential?

We know from IQ tests in children that cognitive ability can be determined early; even in infants, simple tests of mental development (like pulling a string to ring a bell, matching pictures, and sorting pegs by BabyIQ color) can predict mental ability later in life. So, if it’s possible to gauge mental ability of 7-year olds, and if fraternal twins typically score differently on IQ tests, why did the kids in the study perform at the same level? What was special about these 7-year olds?

The answer, of course, was poverty. When researchers compared the IQs of 7-year olds from affluent backgrounds, variation between twins was huge- as expected. Poverty effectively erased an individual’s genetic contribution towards intelligence; thus, twins from low socioeconomic standing bunched together on the IQ spectrum, and twins from high socioeconomic standing fanned out according to their innate genetic potentials.

At 7 years old, what is different between the lives of kids from low and high socioeconomic levels? Is there any one environmental factor that can flip the switch of mental development, and allow poor children to reach their full capacity for intelligence?

Though there’s much debate about the efficacy of early childhood education, kids from poor backgrounds typically have less access to schooling than their wealthier counterparts. They’re not as likely to participate in preschool, and have fewer program options. When poor kids are given the chance to attend preschool, their gains in literacy vastly outpace those of their affluent peers, and the cavernous achievement gap begins to shrink.

If such drastic changes in literacy can be seen as early as 4 years of age, is it possible that differences in environment can affect the IQs of even younger children? Or of babies? Einstein_baby
When do the effects of socioeconomic status really start to define a child’s mental ability? Until two months ago, the best answer we had to this question was 7 years old. Now, a new study has shown it to be much earlier.

Researchers from the Universities of Texas at Austin, British Colombia, and Virginia built on the work from 2003 and compared the mental abilities of 10 month-old twins from families with a range of income, education, and occupational levels. They then waited 14 months, and repeated their analysis of the same children.

At 10 months of age, infants across all socioeconomic levels performed similarly on tests of mental aptitude. (In other words, neither genes nor background had obvious effects on intelligence.) But, at 2 years of age, increases in mental ability were highly dependent on the child’s background. For wealthy children, half of the increase in mental ability was due to genes; for poor children, genes had little or no effect.

Before kindergarten, before preschool, before many of these children had even learned to walk, their mental abilities (and likely their social and emotional IQs) were already being molded by their environments. If not formal education, what’s responsible for early cognitive growth? The researchers speculated that frequent interaction with caregivers helps promote intellectual gains. Because poor children are more likely to be born to single, working mothers, their amount of person-to-person contact may be inadequate for proper development.

Whatever these external factors may be, after only 10 months of life, the environment has started to wrap its fingers around the raw potential of our genetics. Squeeze too tightly, and we languish at the level set by our socioeconomic status. Loosen the grip, and we may have the opportunity to reach our full genetic potential.


1. Socioeconomic status modifies heritability of IQ in young children.Turkheimer E, Haley A, Waldron M, D'Onofrio B, Gottesman II.Psychol Sci. 2003 Nov;14(6):623-8.

2. Emergence of a Gene x Socioeconomic Status Interaction on Infant Mental Ability Between 10 Months and 2 Years.Tucker-Drob EM, Rhemtulla M, Harden KP, Turkheimer E, Fask D. Psychol Sci. 2011 Jan 1;22(1):125-33. Epub 2010 Dec 17.