Annual income drops of 25% or more during formative earning years were tied to worse cognitive function and brain integrity in midlife, a prospective cohort study of Americans showed.

Higher income volatility and more drops in annual income over a 20-year period were associated with lower performance on processing speed and executive functioning tests, and worse microstructural integrity of total brain and white matter, reported Leslie Grasset, PhD, of the INSERM Research Center in Bordeaux, France, and colleagues in Neurology.

The findings were similar when the analysis was restricted to people with higher education, suggesting reverse causation may not explain the results, the team added.

"Over the last decades, income volatility has become more frequent than ever," Grasset told MedPage Today. "Moreover, recent studies have evidenced that it may have negative consequences on different health aspects such as mental health or cardiovascular diseases. However, policies intending to absorb unpredictable income changes are undergoing continuous changes -- mostly cuts -- in the U.S. and other countries."

Earlier studies about income and brain health have looked at income at one point in time without examining long-term changes in income, Grasset pointed out. "Our study is the first one reporting the relationship between income volatility and cognitive function or brain markers," she said. "This relationship from early adulthood to midlife remains relatively underexplored. This paper shows that, beyond having low income at one time point, individuals who experience important income fluctuations over a 20-year period during formative earning years showed lower cognitive performances and worse brain integrity at midlife."

The exploratory study involved 3,287 people who enrolled in the racially diverse Coronary Artery Risk Development in Young Adults (CARDIA) cohort when they were ages 23 to 35. They reported annual pre-tax household income every 3 to 5 years for 20 years from 1990 to 2010, recording it into one of nine brackets that started at $0–$2,500 and ended at $75,000+. The bracket midpoint was chosen as the participant's income for the year.

The researchers assessed each participant's income volatility, defined as how often inflation-adjusted income dropped from 1990 to 2010 and the percent change in income. Participants fell into three groups:

1,780 people did not have an inflation-adjusted income drop

1,108 had one drop of 25% or more

399 people had two or more drops of 25% or more

At the end of the 20-year period, all participants had cognitive assessments. A subset of 707 people also had magnetic resonance imaging (MRI) scans in 2010 to measure brain volume and assess microstructural brain integrity using fractional anisotropy.

In general, participants with more income drops were more likely to be black, less educated, unmarried, and smokers, with lower income and to be unemployed at baseline. They also had more elevated depressive symptoms, a higher body mass index and systolic blood pressure, and were less physically active.

After adjusting for covariates -- including body mass index, blood pressure, medication, smoking status, physical activity, and other factors -- income volatility was tied to several measures of cognitive function -- specifically, worse performance on processing speed (measured by the Digit Symbol Substitution Test; β −1.09, 95% CI −1.73 to −0.44) and executive functioning (measured by the Stroop test; β 2.53, 95% CI 0.60–4.39). Income volatility was not related to verbal memory.

Income volatility also was linked to lower microstructural brain integrity (total brain β −0.75, 95% CI −1.29 to −0.18; white matter β 0.003, 95% CI −0.005 to −0.0009).

Two or more annual income drops were associated with poorer scores on processing speed (β−3.74, 95% CI−5.35 to −2.12) and executive function (β 8.04, 95% CI 2.94–13.31), as well as lower total brain volume (β−0.88, 95% CI −1.53 to −0.23), total brain microstructural integrity (β −1.59, 95% CI −2.96 to −0.20), and white matter integrity (β −0.008, 95% CI−0.01 to −0.003).

These outcomes may represent markers of vulnerability to cognitive decline and brain aging, Grasset and colleagues noted.

"Possible explanations for the study's observations include increased adoption of unhealthy behaviors (though results remained relatively unchanged after accounting for many of these factors), increased exposure to detrimental environmental and psychosocial stressors, and neurobiological changes related to decreased engagement in challenging mentally stimulating activities or lower social support," observed Joel Salinas, MD, MBA, MSc, a neurologist at Massachusetts General Hospital in Boston, in an accompanying editorial.

This research might capture the attention of primary care providers, public health agencies, economists, policymakers, and activists before neurologists, he noted. "But the findings should give all health care providers cause for pause. These results are part of a larger ongoing story that points to social conditions as inseparable from disease and poses a critical question for each neurologist and for our field: What is our responsibility in addressing social determinants of health?"

The study had several limitations, Salinas added. As an observational study, it cannot show causality. Cognitive and MRI outcomes were not assessed at baseline, and income was self-reported and recorded into brackets.

Using the midpoint of an income bracket may have resulted in a loss of precision and misclassification, Grasset and co-authors noted. Large income changes within income brackets could not be seen, and small changes occurring close to bracket thresholds may have been detected as income category changes.