A study published in Wellcome Open Research has uncovered evidence that exposures experienced by grandparents, including smoking and economic conditions at the time of their birth, may be associated with reduced IQ scores in their grandchildren.
The research suggests potential lasting impacts across multiple generations of certain harmful environmental factors.
Key facts:
- The study found associations between grandfathers’ smoking and lower IQ scores in grandchildren at age 15, especially in granddaughters.
- Grandmothers’ smoking during pregnancy was linked to reduced performance and verbal IQ at age 8, particularly in grandsons.
- More favorable economic conditions, as measured by higher GDP, at the time of a grandmother’s birth was associated with lower IQ in grandchildren. This effect was stronger in granddaughters.
- Associations differed between the maternal versus paternal lineage and between grandsons versus granddaughters, implying sex-specific effects.
- The findings raise questions about the mechanisms underlying apparent transgenerational effects of ancestral exposures on descendants’ cognitive development.
Source: Wellcome Open Res.
Transgenerational Effects in Humans
In recent years, scientists have uncovered evidence that environmental exposures experienced by ancestors may have effects that reach multiple generations down the family line.
Such transgenerational effects have been observed in both animals and humans for exposures like nutrition, smoking, and toxic chemicals.
Now, new findings from the Avon Longitudinal Study of Parents and Children (ALSPAC) based in Britain provide some of the first data pointing to potential lasting cognitive impacts on grandchildren related to harmful exposures among grandparents.
The study found associations between grandfathers’ smoking history, economic conditions during grandmothers’ pregnancies, and reduced IQ measured in their grandchildren decades later.
“If substantiated in other studies this raises the question as to what the mechanism might be,” said lead author Jean Golding, Professor of Pediatric and Perinatal Epidemiology at the University of Bristol.
“We have argued elsewhere that at present we are largely ignorant of the causal pathways underpinning this and similar intergenerational responses to grandmaternal smoking in pregnancy.”
Grandparent Smoking Linked to Lower IQ
The researchers utilized data on over 13,000 children from ALSPAC, which has followed the health of children born in 1991-92 from pregnancy onward.
When the mothers were pregnant back in the early 90s, they reported information about their own parents, the study children’s grandparents.
This included details about the grandparents’ smoking history and the economic conditions around the time of the grandparents’ birth, based on Gross Domestic Product (GDP) data.
The researchers analyzed these ancestral exposures in relation to the grandchildren’s IQ measured at ages 8 and 15 years.
After adjusting for potential confounding factors, the study found that grandfathers’ smoking history was associated with lower overall IQ scores at age 15, especially in granddaughters.
For grandmothers’ smoking, the links were a bit different – grandmothers smoking during pregnancy was associated with lower performance and verbal IQ at age 8, particularly in grandsons.
Higher GDP at Grandmother’s Birth Linked to Lower IQ
In addition to smoking, the researchers uncovered surprising associations between economic conditions at the time of ancestral births and grandchildren’s cognitive abilities.
Specifically, higher GDP (a measure of economic prosperity) during a grandmother’s birth year was linked to lower IQ scores in their grandchildren.
This effect was strongest for associations between paternal grandmothers and granddaughters.
The reasons for the GDP associations are unclear, but the researchers speculate they could reflect broader implications of economic development trends over the 20th century.
For example, times of high GDP may be linked to increased industrialization and exposure to air pollutants.
“Of the 96 tests carried out, 13 demonstrated an association between increasing levels of GDP at the time of a grandparent’s birth and a reduction in the IQ of their grandchild,” the authors wrote.
“This was particularly true for the years of birth of the grandmothers as opposed to grandfathers (10 and 3 respectively).”
Sex-Specific Effects Across Generations
Intriguingly, many of the smoking and GDP associations differed based on the grandchild’s sex in relation to the sex of the ancestor exposed.
For example, paternal grandmother’s smoking was linked to reduced performance IQ specifically in grandsons, while maternal grandfather’s smoking was associated with lower IQ scores particularly in granddaughters.
Similarly, associations between higher GDP at time of grandmother’s birth and lower grandchild IQ were stronger when tracing the paternal versus maternal lineage, and for granddaughters compared to grandsons.
According to the researchers, these sex-specific transgenerational effects align with other studies showing sex differences in associations between grandparental smoking and grandchild outcomes like BMI and asthma risk.
The differing male versus female lines of transmission suggest possible explanations involving sex chromosomes or imprinting effects, but more research is needed to elucidate the biological mechanisms.
Epigenetics: A Plausible Explanation?
What could explain the apparent ability of ancestral exposures to influence cognitive development generations later?
One leading possibility that scientists have proposed involves epigenetics – molecular changes that affect gene activity without altering the DNA sequence itself.
The hypothesis is that exposures like smoking might cause epigenetic modifications, like DNA methylation or histone alterations, in the germline cells that later give rise to grandchildren.
These epigenetic marks may then persist from one generation to the next and affect patterns of gene expression during development in subsequent generations.
Some experimental studies in animals provide support for this idea.
For example, rodent experiments have shown that nicotine exposure can alter germline DNA methylation patterns, with behavioral and metabolic effects observed in offspring and grand-offspring generations not directly exposed.
While more speculative in humans, the new findings are consistent with the possibility that epigenetic mechanisms may underlie apparent transgenerational effects of grandparental smoking on cognitive outcomes.
Further research incorporating multi-generational molecular analyses will be important to evaluate this hypothesis directly.
Further Research on Transgenerational Effects Needed
While uncovering intriguing associations, the authors acknowledge key limitations of the study that prevent inferring causality or definitive mechanisms at this stage.
Importantly, the findings need to be replicated in other population datasets to confirm the results, which currently offer only an initial signal.
The study also relied on parents’ reports about their parents’ smoking history and economic conditions decades earlier, which may not be fully accurate.
Additionally, details on potentially influential factors like grandparents’ education levels were not available.
The numbers of grandchildren analyzed, while larger than prior studies, were still small in the context of detecting transgenerational effects.
Nonetheless, the ALSPAC analysis represents an important step toward understanding potential cognitive impacts that may reach across multiple generations.
Going forward, studies that directly track family lineages over time, incorporate molecular data, and experimental animal models will be key to clarifying the existence and mechanisms of transgenerational effects relevant to human disease and cognition.
In summary, this new research provides initial population evidence that harmful exposures like smoking among ancestors may have subtle yet lasting consequences for descendants far down the family line.
Though requiring more verification, the findings imply that the environments and behaviors of our grandparents and great-grandparents may continue to shape the cognitive trajectories of current and future generations.
- References
- Study: Grandchild’s IQ is associated with grandparental envrionments prior to the birth of parents
- Authors: Jean Golding et al. (2020)
