Due to limitations on traditional family designs for studying family income, researchers have relied on within-individual variation or extended family studies as quasi-experimental approaches. Because of the assumptions and limitations required in the various approaches, converging evidence is required before researchers can infer causal associations between risk factors and outcomes (Rutter et al., 2001
). The current study tested the causal hypothesis that family income has a specific, environmentally-mediated causal effect on offspring CPs. This was done in an attempt to replicate the results of smaller randomized (Morris & Gennetian, 2003
) and quasi-experimental studies (Costello et al., 2003
). This is important because the designs of the studies are subject to different threats to validity. We also wanted to determine if the results of the previous studies of local samples generalize to the diverse U.S. population. Identifying the causal risk factors for early CPs is crucial because they predict adolescent and adult criminal activity (Lahey, Waldman, & McBurnett, 1999
; Moffitt, 2003
Quasi-experimental studies cannot prove causality (Shadish, Cook, & Campbell, 2002
), but the current study, which used rigorous approaches to control for both measured and unmeasured confounds that may account for the association between family income and offspring adjustment, supports a causal inference of the role of family income in the development of offspring CPs, especially for males. The analyses controlled for family and maternal characteristics (history of delinquency and alcohol abuse/dependence) that were excluded from previous studies of the NLSY. This is an important advance, given that parental psychopathology is the strongest predictor of offspring psychopathology (e.g., Rutter et al., 1997
). The analyses also utilized the family-based structure of the NLSY study to compare cousins, all siblings, and full siblings to account for unmeasured genetic and environmental factors that could act as selection factors. The current study is the first study to control for both maternal and
paternal selection factors by restricting the analyses to compare differences only among full siblings. Although the findings from the restricted sample that only included full siblings cannot be generalized to children in all types of families (e.g., mixed families), the results imply that when genetic confounds are controlled (Rowe & Rodgers, 1997
), the evidence still suggests that family income exerts a specific independent influence on male child CPs.
The model fitting suggests, consistent with Blau (1999)
, that controlling for both measured and nonmeasured selection factors greatly reduces the magnitude of the statistically significant association between family income and the child’s CPs. illustrates the nonlinear inverse association between family incomes and CPs and shows how the use of statistical and methodological controls attenuates the association. It is important to note, however, that the present quasi-experimental analyses yield a very conservative
estimate of the causal impact of family income on offspring CPs for a number of reasons. First, the analyses included many factors that could mediate the association between family income and offspring CPs (e.g., maternal alcohol problems), but that also could be a result
of exposure of poverty in the adult generation. Thus, controlling these covariates could “over-control” and artificially attenuate parameter estimates for family income. Second, the reduced estimates of family income within families could be partly or wholly due to the marked restriction in the range of income that is typical within families. The use of within-family estimates is a powerful tool for exploring causal risk mechanisms, but it is likely that they minimize the estimates of income effects by a considerable extent. The variability within a nuclear family is much lower than the variability in the general population (as indicated by the high intraclass correlations), so there may not be enough variability between siblings to accurately assess the full influence of income. Indeed, the fact that the present conservative within-family analyses support the hypothesis of an independent causal effect of income is quite surprising. The results from the models fit to only full siblings not only depend on the limited variability in income within a nuclear family, they are based on intact families with higher levels of income on average than nonintact families. The goal of the current analyses was to explore whether there was any independent association between family income and offspring CPs when controlling for as many selection factors as possible. As such, the present findings provide a very conservative estimate of the association of family income and offspring CPs.
The results suggest the association between family income and childhood conduct problems remain for boys but not girls when using within-family comparisons. Costello et al. (2007) found that parental supervision partially mediated the effect of income on offspring disruptive behaviors. Considering that several studies have found parental supervision to be more strongly associated with CPs for boys than for girls (e.g., Storvoll & Wichstrom, 2002
; Martens, 1997
), it is possible than gender differences in the effects of supervision could explain the gender differences. The measure of CPs may have also influenced the findings; the assessment did not include items that might better index disruptive behaviors in females, such as relational aggression (e.g., Zoccolillo, 1993
The present analyses suggest a non-linear association between family income and offspring CPs. Variations within the lowest range of incomes had much less of an effect than variations across the moderately low to high range of the income distribution. This was not due to the specific transformation technique that was used, the specific measure of family income, or the comparison group (unrelated individuals, cousins, and siblings). The current findings of a non-linear effect of income both confirm (e.g. Blau, 1999
) and contradict previous research (Dearing, McCartney, & Taylor, 2001
). Differences in findings may stem from the use of different definitions of income or the use of different measures of behavior problems.
The current study complements previous quasi-experimental studies of family income on offspring adjustment and expands knowledge about the role of family income by balancing concerns about generalizability and the specificity of findings. With respect to generalizability, the current analysis used the offspring of women from a nationally representative sample of households in the United States, adjusted for the sampling design, to assist in the population-based estimation of income effects. The present study also utilized a measure of offspring CPs and family income that were the average across the ages of 4 to 11 years old, which enabled the study to include many more offspring of the women than previous research using the NLSY dataset and reduced the bias present in previous studies that relied on offspring mostly born to very young and financially disadvantage mothers. The use of the average scores did not, however, hinder the interpretation of our findings, as previous research has shown that the measures are stable, reliable across childhood, and valid (D’Onofrio et al., 2007
; Lahey et al., 2006
). Finally, using multiple imputation and other advanced statistical methods (Schafer, 1997
) to accommodate missing data meant that the findings are highly generalizable even if data were not missing completely at random.
With respect to specificity, the analyses were restricted to a focused outcome and children’s CPs. Previous studies have typically relied on global measures of child mental health that either combined internalizing and externalizing problems (e.g., the full Behavior Problems Index) or analyzed all externalizing problems. Previous research based on the CNLSY sample has suggested that these dimensions of childhood adjustment, although correlated, are differentially sensitive to early risk factors (D’Onofrio et al., 2008
; D’Onofrio et al., 2007
). Moreover, there is evidence that changes in income are related in different ways to different types of mental health problems (Costello et al., 2003
). The current results strengthen and clarify previous research that was ambiguous because the child measures combined externalizing and internalizing problems in a single scale. Further research is necessary to explore whether family income differentially predicts various aspects of psychopathology.
The existing literature on whether early childhood is a particularly sensitive period for the effects of family income is inconsistent (e.g., Hau & Matsueda, 2006
; Costello et al., 2003
). Our findings suggest family income early in childhood (0–3 years old) did not uniquely predict childhood CPs independent of family income throughout the age range (0–11 years old). Further research, however, is needed to further explore whether early childhood is a particularly sensitive period, because the null findings in the current research could be due to the limited number of offspring who experienced great variability in exposure to family income from early to late childhood.
There are numerous threats to the validity of the findings in the current manuscript. All quasi-experimental studies are limited in their ability to identify causal processes (e.g., Shadish, et al., 2002
). Using within nuclear family variation provides a much stronger basis for causal inferences than the comparison of cousins or unrelated individuals, but this approach can detect associations only to the extent that that families that experience changes in their inflation-adjusted income from the childhood of one sibling to the childhood of another sibling. Furthermore, this approach is valid only if families that experience large changes in income are similar to families who do not experience such large changes in income. Fortunately, the comparison of cousins, which was also included in the current analyses, does not require variation in income over time within nuclear families; rather variation in income is required among adult siblings. As such, the use of sibling and cousin comparison provides converging evidence on the effects of income within the study.
In addition, the present analyses, which were focused on ruling out alternative explanations for the association between family income and offspring CPs, did not specifically test for mediators of the relation. Future quasi-experimental studies will need to test various mediation models, such as the home environment, neighborhood risk factors, and parenting quality (Duncan & Brooks-Gunn, 2000
), in addition to the role of non-resident and cohabitating fathers (Manning & Lichter, 1996
The current analyses, similar to all quasi-experimental design, is unable to rule out every possible confound (Shadish et al., 2002
). The methodological approach that we utilized completely controlled for genetic risk that are passively passed from parents to offspring, a major limitation of most previous research on family income (e.g., Rowe & Rodgers, 1997
). That is, the comparison of full siblings accounts for 100% of the passive genetic effects because meiosis randomly transmits alleles of polymorphic parental genes across siblings (Rutter, 2007
). Sibling comparisons, however, do not completely account for all possible genetic and environmental differences among the children that could influence family income. It is possible each offspring’s behavior could have influenced their parents’ earnings when they were a child (e.g., by leading a parent to quit work to care for a difficult child). Only experiments based on random assignment can fully rule out all such active or evocative confounds. Fortunately, because the threats to the validity of the present study are different from the threats to other previous studies using different methods (e.g., Costello et al., 20003
; Morris & Gennetian, 2003
) that also found evidence that higher family income protects offspring from CPs, the present findings add support to the inference that family income influences risk for CPs through causal environmental processes. Collectively, these studies provide a strong basis for conducting randomized controlled trials to test the potential benefits of programs to increase family income.
Except for the sex of the offspring, we did not explore whether characteristics of the individual, mother, or family moderate the association between family income and offspring externalizing behaviors. Future research using the NLSY and other datasets will need to explore whether certain offspring or families are more sensitive or resilient to the effects of family income. As such, the current analyses suggest that interactionist models (Conger & Donnellan, 2007
) may be most useful in understanding SES, as both processes appear to be present.
Overall, the current results are consistent with a causal association between family income and offspring CPs, especially for male offspring. These findings suggest that increases in family income during childhood would be expected to decrease offspring CPs. The results of the current and previous studies have important implications for the types of financial interventions that may be necessary to alter childhood CPs. In particular, the non-linear association between family income found in the present analyses and some previous reports suggest that small increases in family income that do not raise the family out of the low range of incomes may not be sufficient to reduce childhood CPs. For the reasons described above, however, our methods may underestimate the effects of income on CPs. Therefore, the potentially non-linear association between family income and childhood CPs is an important topic for future research, as it could set boundaries of the kinds of changes in social and economic policy that could be expected to decrease childhood CPs.