New research published in autism research found that the age at which maternal grandparents had children was associated with the likelihood that their grandchildren would develop autism. Research shows that this association varies widely by race and ethnicity. These changes suggest that environmental and social factors act in parallel with biology to influence child development over multiple generations.
Autism spectrum disorder is a neurodevelopmental disorder characterized by repetitive behaviors and difficulties with social communication. In recent decades, a rapid increase in the prevalence of autism has been observed. In California, the percentage of children diagnosed with the disease has increased dramatically from the early 2000s to recent years. During this period, diagnosis rates for historically underrepresented minority groups exceeded those seen among white children.
As diagnostic rates have changed, demographic patterns in family planning have also changed. In the United States, the average age of parents at birth has steadily increased. Previous research has established a strong link that older parents are more likely to have a child on the autism spectrum.
Recent evidence extends this timeline backwards, showing the age of the grandparents. When grandparents conceive their parents, their age can have biological and social effects on their grandchildren. To understand this multigenerational pattern, public health researcher Ting Chou and colleagues at the University of California, Los Angeles, launched a large-scale study.
Previous research on grandparent age has focused almost exclusively on white European populations. The researchers wanted to see if the same age pattern emerged within a highly diverse population. They hypothesized that studying different racial and ethnic groups could help determine whether these age associations are universal biological characteristics or markers of different environmental conditions.
The biological mechanisms behind multigenerational health conditions are still being actively researched. One possibility is changes in the germ cells, which are the sperm and eggs. Environmental exposures and the natural aging process can change epigenetic marks. These marks act as chemical control systems that turn genes on or off without changing the underlying genetic code.
Aging can also lead to the accumulation of damage to mitochondria, the energy-producing structures within cells. Children inherit mitochondria only from their mother’s eggs. While still in the mother’s womb, a female fetus develops all the eggs it will produce. Because of this timeline, the grandmother’s age and environment during pregnancy can directly impact the health of the egg cells that will eventually become her grandchildren.
An aging grandfather can pass on new genetic mutations and epigenetic changes through his sperm. Unlike women’s eggs, men’s sperm reproduce continuously after puberty. This constant replication increases the opportunity for cellular errors to accumulate as we age.
To explore these possibilities, researchers analyzed California birth records. The researchers linked the health records of children born between 2001 and 2019 with the birth records of their mothers born between 1983 and 2001. They also incorporated diagnostic data from the California Department of Developmental Services to identify cases of autism.
This complex data linkage created a study population of more than 1.7 million mother-infant pairs for grandmother analysis. Within this large group, researchers identified approximately 28,000 children diagnosed with autism. They then went back in time to determine the exact ages of the maternal grandmother and maternal grandfather when the mother was born.
The researchers categorized the grandparents’ ages into four categories. They defined young grandparents as those between the ages of 18 and 24. The reference group consisted of grandparents aged 25 to 29 years. We also created categories for ages 30-34 and 35-55.
They used a statistical model to compare the odds of being diagnosed with autism among grandchildren based on these different age groups. Their calculations took into account the child’s and mother’s year of birth, the child’s gender, and the number of previous pregnancies experienced by the grandmother.
Overall, the researchers found that grandchildren were slightly more likely to be diagnosed with autism if their maternal grandparents were unusually young or relatively old when the mother was born. However, these broad averages masked clear differences between different racial and ethnic communities. The researchers observed a particularly pronounced U-shaped statistical curve among white grandparents.
For white grandmothers and grandfathers, both the youngest and oldest age groups were associated with increased odds of autism in their grandchildren. Mathematics relationships looked different among Hispanic grandparents. Grandchildren are more likely to have autism only if their Hispanic grandmother or grandfather is in the oldest category.
When looking at other demographic groups, the pattern changed again. Among Asian-Pacific Islander families, the odds of autism were higher only among older grandmothers. The researchers noted that black families had decreased odds of documented autism in relation to young grandmothers. At the same time, older black grandfathers were found to be associated with higher rates of autism in their grandchildren.
The study authors suggest that these racial and ethnic differences indicate differences in the underlying mechanisms. Among white populations, having children at an exceptionally young age is often correlated with lower socio-economic status. Challenges associated with limited resources can ripple across generations and impact the health of grandchildren.
The opposite pattern seen among young black grandmothers may be explained by a phenomenon known as birth bias. This statistical concept suggests that the most vulnerable fetuses may not survive to birth. If young, socio-economically disadvantaged grandmothers experience high rates of miscarriage, the children born may represent a particularly resilient subset, artificially lowering the autism rates observed in the data.
In contrast, older grandparents from most demographic groups showed a positive association with autism in their grandchildren. This consistency is consistent with biological theories of cell damage or accumulation of genetic mutations. Environmental factors disproportionately affect minority communities and, in combination with older age, can also negatively impact reproductive health.
Historically, Black and Hispanic people in California have been more likely to face occupational hazards, higher levels of economic stress, and systemic discrimination. Chronic stress and exposure to toxic environments can affect fetal development. When a grandmother experiences these difficulties during pregnancy, changes occur in the developing eggs within the woman’s fetus, which can ultimately affect her grandchildren.
The researchers acknowledged some caveats in their analysis. Total reliance on birth and diagnostic records means that subtleties of family life remain unmeasured. For example, state databases did not include comprehensive information on lifestyle habits such as smoking and drinking. Researchers also lacked data on family history of mental illness.
The investigation was strictly limited to the mother’s genetic lineage. The researchers focused on maternal grandparents because paternal identification information in old birth records has historically been less complete. This data gap precluded a comparable analysis of paternal grandparents and their generational effects. Data availability also forced the researchers to exclude older mothers born before electronic records began in 1983.
Future studies should incorporate detailed biological, social, and environmental data. Expanding this study to diverse populations outside of California could help reveal how different social contexts interact with genetic inheritance. Tracing both your maternal and paternal ancestry lines will give you a more complete picture of your family’s health timeline.
The study, “California Multigenerational Study of Age, Race, and Ethnicity of Maternal Grandparents in Autism Spectrum Disorders,” was authored by Ting Chow, Qi Meng, Jingyuan Xiao, Karl O’Sharkey, Zeyan Liew, and Beate Ritz.
