Childhood abuse was linked to slower thinking decades later, and inflammation explained only a sliver of it

Hard experiences early in life, such as abuse or neglect, have long been linked to poorer memory and thinking in middle and old age. What is much less clear is how such experiences could leave a mark that lasts for decades. One popular idea is inflammation, the body's alarm response: long-lasting stress is known to raise inflammation, and chronic inflammation is thought to harm blood vessels and nerve cells in the brain.

A team at Pennsylvania State University tested that idea in two large groups of adults. The result was mixed. Childhood abuse and neglect were clearly linked to lower scores on thinking tests, but inflammation accounted for only a small part of that link, at most about 5%. The study compares people at one point in time and relies on their memories of childhood, so it can show links but not prove that one thing led to another.

What they did

The researchers used two existing studies. The first was UK Biobank, a large British study of people who were 40 to 69 years old when they joined; depending on the test, up to 132,068 of them could be included. The second was a much smaller American study called MIDUS (Midlife in the United States): 276 people aged 25 to 75 with blood inflammation measurements, and 209 with measurements of gene activity in their white blood cells. Because too few people from other ethnic groups were available for a fair comparison, both analyses were limited to White participants.

Participants answered questions about childhood physical abuse, emotional abuse, neglect and (in the US group) economic hardship, and about loneliness and social isolation as adults. They also took thinking tests covering memory, reasoning, mental processing speed and planning.

From blood samples, the team measured five inflammation markers, including C-reactive protein (CRP), a protein the liver releases during inflammation, and ICAM-1, a "sticky" molecule on blood vessel walls that helps immune cells cross into tissues. In the US group they also looked at a set of genes known to switch on in white blood cells under long-term stress.

Then they used a statistical method called mediation analysis, which asks: of the link between a stressor and a test score, how much travels "through" a third measure, here inflammation? The results were adjusted for age, sex, income or deprivation, physical activity, body weight and depressive symptoms, and corrected for the large number of comparisons.

What they found

• Abuse and neglect went with lower scores. Across both cohorts, childhood physical and emotional abuse were the most consistent predictors of poorer thinking. In UK Biobank, childhood neglect showed the strongest links of all, especially with a timed symbol-matching test and with reasoning.
• A small share through inflammation. In UK Biobank, only three pathways held up after the correction for many comparisons. Higher CRP accounted for about 1.5% of the link between childhood physical abuse and reasoning, and about 1.1% of the link between emotional abuse and reasoning. ICAM-1 accounted for about 5% of the link between emotional abuse and mental speed.
• Nothing firm in the smaller group. In the US group, none of the blood markers showed a significant pathway. A stress-related gene called TNF showed two weak signals, but they did not survive the correction and pointed in an unexpected direction.
• Age made little difference. The share explained by inflammation did not differ between younger and older participants. For one speed test, the link with childhood adversity was weaker in people around 65 than in those around 45.

What to keep in mind

• A snapshot, not a story over time. Inflammation and thinking were measured at the same time, so the study cannot tell what came first. The authors are careful to say the results are "consistent with" partial mediation rather than proof of it.
• Memories of childhood. Abuse and neglect were reported by the participants decades later, which the authors note can introduce recall bias.
• Small US group. The authors say the MIDUS samples were too small to reliably detect effects, so the absence of findings there means little either way.
• Only White participants. The results may not apply to other groups.
• Different tests in each country. The two cohorts used different thinking tests, which limits direct comparisons. The way the stressors were grouped was the team's own choice, and a different grouping could give different numbers.

Why it matters

If inflammation were the main bridge between early hardship and later thinking problems, calming inflammation might be a way to break the chain. This study suggests the bridge is mostly elsewhere. The authors point to other likely routes: effects of early adversity on how the brain develops, on education and opportunities to learn, on mental health and on stress hormones. Their conclusion is that inflammation is "linked to only part" of the story, and that understanding how childhood experiences shape the aging brain will need all of these pathways studied together.