There's also some controversy about the precise extent to which the course of early development determines cognitive ability, with some researchers (for example Herrnstein and Murray in The Bell Curve) arguing that differences in adult intelligence between populations are to a significant extent genetically based. I'm not going to try to say anything about the wider issues here, but this paper by Sampson, Sharkey and Raudenbush is an important contribution.
They key idea behind the research is an attempt to study the neighbourhood as a determinant of cognitive ability. There are several good reasons for this, reviewed in the paper, including that neighbourhood poverty is associated with inconsistent parenting, and poor mental health in parents. There are also ways in which marginal and violent neighbourhoods might be expected to compromise quality of speech communication, and in other ways limit opportunities for learning. The authors therefore hypothesised that "residing in a severely disadvantaged neighborhood cumulatively impedes the development of academically relevant verbal ability in children" (p 846).
There are non-trivial methodological problems in attempting to measure effects here. Some relate to analysis. Controlling for income, for example, while sensible in some ways amounts to hypothesising that neighbourhood doesn't itself have an effect on income, or at least reduces the chance of detecting multiple determinants of different cognitive ability. The present study attempts to deal with this in ways I’ll get to in a moment. There are also problems with selection of subjects, since the majority of studies are cross-sectional. The authors of this study opted to compare within-individual changes as related to stability or change in neighbourhood as a way of avoiding the limitations of cross-sectional approaches.
The study followed 2000+ children, living in Chicago, between the ages of 6–12. The subjects, along with their caretakers, were followed when they moved for up to 7 years. As the authors put it:
Approximately 17% of black children not living in disadvantage in 1995 moved to a disadvantaged neighborhood some time between 1995 and 2002, and 42% of the population of black children living in disadvantaged neighborhoods in 1995 moved to a nondisadvantaged neighborhood between 1995 and 2002. We exploit these ''within-individual'' changes to estimate the causal effect of moving (p 846).
Leaving out a lot of important detail, the study proceeded as follows neighbourhoods were classified into levels of disadvantage using Census data, and subjects' trajectories classified according to a series of 'waves' giving a number of sequences (e.g. disadvantage, nondisadvantage or nondisadvantage, disadvantage). Baseline disadvantage (wave 0) was not regarded as a 'treatment' but subsequent waves were. The different trajectories were related to a measure of cognitive ability combining the Wechsler Intelligence Scale for Children vocabulary test and the Wide Range Achievement Test reading examination. Subjects were selected from nearly 35,000 households, and interviewed and assessed three times over a seven year period.
For various reasons the published analysis, concerning 'concentrated disadvantage' focuses on the African-American subjects. (Concentrated disadvantage was defined as neighbourhoods falling in the bottom quartile of a distribution factoring in six elements of the Census data.)
The most important finding, and the one noted in the title of this paper, is represented in this graphic. It compares cognitive ability scores of children who started out in neighbourhoods with concentrated disadvantage and remained there for both waves of the analysis with those of children who started in the same position but moved away from concentrated disadvantage for wave 2. Again quoting from the paper "Scores for black children who lived in such neighborhoods declined sharply relative to the average rate of growth in the sample as a whole, so that by wave 2, their verbal ability scores are well below the mean and_4 points below those of black children who do not experience the treatment" (p 851).
This is mixed news. It's lousy that it seems as though early disadvantage is so harmful. (The authors are cautious about extrapolating without additional justification from their results, but it's still lousy, and only more so if extrapolation is warranted.) On the other hand, it suggests where to focus at least some policy attention: fix the places kids grow up.
The paper is also currently available on Sampson's website here.
Sampson, R.J., Sharkey, P., Raudenbush, S.W. (2008). From the Cover: Inaugural Article: Durable effects of concentrated disadvantage on verbal ability among African-American children. Proceedings of the National Academy of Sciences, 105(3), 845-852. DOI: 10.1073/pnas.0710189104