Genetic factors, not maternal smoking, could be behind lower academic achievement in children, says a new US-Swiss study.
Maternal smoking during pregnancy (SDP) has been consistently linked with numerous birth complications and problems in the offspring, including unsuccessful pregnancy outcomes, lower intellectual abilities, lower academic achievement (AA), more inattention and hyperactivity problems, and increased antisocial behavior.
SDP is correlated with psychosocial risks, such as early age at childbearing; parental history of psychopathology and antisocial behavior, low socioeconomic status, poor parenting, and ongoing exposure to parental substance use.
These correlated risks could confound the association between SDP and offspring adjustment. A number of researchers have also argued that genetic confounds may explain the associations between SDP and offspring adjustment.
Researchers led by Brian D'Onofrio, Department of Psychological and Brain Sciences, Indiana University, used a number of statistical and quasi-experimental approaches to explore the underlying causal mechanisms responsible for the association between maternal SDP and offspring AA.
They build on a previous study which compared siblings differentially exposed to SDP on school grades using offspring born in Sweden between 1983 and 1987. " We use a larger sample to explore the association between SDP and grade average (using data on every individual born in Sweden between 1983 and 1991), include scores from a standardized assessment of mathematic proficiency given to all students in Grade 9 throughout the country, explore the possible confounding role of measured maternal and paternal characteristics, and utilize multiple quasi-experimental designs to help specify the causal mechanisms through which SDP is associated with lower AA," the researchers said in their findings published in the Child Development journal.
The association between SDP and maternal, paternal, and familial risks were based on a woman's history of ever smoking across her pregnancies. SDP was associated with maternal occupational status; compared to women in unskilled blue collar jobs, SDP was associated with lower likelihood of working in white collar jobs. SDP was also associated with greater likelihood of being in jobs that were difficult to categorize or being unemployed. SDP was also associated with maternal highest level of educational achievement; women with a history of SDP were less likely to receive secondary and higher levels of education. Being convicted of a criminal offense was also associated with history of SDP. The same general pattern held for paternal risks. SDP was associated with occupational status, highest educational level achieved, and history of criminal offenses in fathers.
The study combined two general quasi-experimental approaches to study the mechanisms through which SDP influences offspring AA. The study relied on the historical decline in SDP, due in part to public health campaigns to deter pregnant women and women of childbearing ages from smoking, to explore whether variations in SDP by the same mother were associated with differences in her offspring's AA. The study also utilized a large family study, including a mixture of sibling and cousin types, to compare relatives who differ in their exposure to SDP and who vary in the degree to which they share genetic risk associated with SDP. By combining these two approaches, the current study sought to pull apart the co-occurring genetic and environmental risks associated with SDP.
The results provide support for two main conclusions. First and foremost, the results strongly suggest that SDP does not cause offspring to have lower AA. " We compared full siblings who were differentially exposed to SDP, an approach that accounts for the genetic factors and environmental factors that siblings share. If SDP causes offspring to have lower AA, perhaps through the neurotoxic effects of exposure to prenatal nicotine, then a child exposed to SDP would be expected to have poorer AA than his or her sibling who was not exposed. For both school grades and mathematics proficiency, however, full siblings differentially exposed to SDP did not differ in their levels of AA. The results strongly suggest that familial risks correlated with SDP, and not the specific effects of SDP, are responsible for the lower AA found in offspring whose mothers smoked during pregnancy," the journal article said.
The second conclusion involves the underlying processes by which SDP is associated with AA. The results suggest that genetic factors passed from parents to their offspring (passive gene-environment correlation) account for at least part of the statistical association. Genetic factors are implicated because the degree to which relatives were genetically correlated moderated the association between SDP and AA when comparing types of cousins and types of siblings.
The fact that SDP is associated with lower AA in half siblings provides compelling evidence that researchers must explore the role of fathers when studying the putative effects of SDP. Although most research on SDP has not included measured characteristics of the fathers, the current results indicate that characteristics of fathers, whether due to genetic risk passed down to the children or psychosocial risk factors, help explain some of the association between SDP and offspring AA.
Finally, it is important to emphasize that the current results do not suggest that SDP has no impact on offspring adjustment, the researchers warn. "The results are limited to measures of AA at age 15 only. The findings for birth weight suggest, as have previous genetically informed and other quasi-experimental studies, that SDP has a specific effect on offspring birth weight that is environmentally mediated. SDP appears to impact particular outcomes more strongly than others, particularly poor pregnancy outcomes. Reducing SDP, therefore, remains a major public health issue."