Race, Ethnicity, Family Socioeconomic Status, and Children’s Hippocampus Volume

Introduction: The hippocampus has a significant role in memory, learning, and cognition. Although hippocampal size is highly susceptible to family socioeconomic status (SES) and associated stress, very little is known on racial and ethnic group differences in the effects of SES indicators on hippocampus volume among American children. Purpose: This study explored the multiplicative effects of race, ethnicity, and family SES on hippocampus volume among American children. Methods: Using data from the Adolescent Brain Cognitive Development (ABCD), we analyzed the functional Magnetic Resonance Imaging (fMRI) data of 9390 9-10 years old children. The main outcome was hippocampus volume. The predictor was parental education. Subjective family SES was the independent variable. Age, sex, and marital status were the covariates. Racial and ethnic group membership were the moderators. To analyze the data, we used regression models. Results: High subjective family SES was associated with larger hippocampus volume. This effect was significantly larger for Whites than Black families. Conclusions: The effect of subjective family SES on children’s hippocampus volume is weaker in Black than White families.

Although a growing literature has documented a link between high SES and children's brain development, most of the existing knowledge is on a select few brain structures such as the amygdala (Evans et al., 2016;Javanbakht et al., 2016;Javanbakht et al., 2015;Kimberly G Noble et al., 2015). For example, most research on the effects of trauma, stress, adversities, and SES are on amygdala structure and function (Evans et al., 2016;Javanbakht et al., 2016;Javanbakht et al., 2015;Kimberly G Noble et al., 2015). While overall effects of SES on both volume and function of the amygdala are well established (Evans et al., 2016;Javanbakht et al., 2016;Javanbakht et al., 2015), some recent evidence has documented diminished effects of SES on amygdala for Black than White children (Assari, Boyce, & Bazargan, 2020b). However, less is known on group differences in the effect of SES on hippocampus www.scholink.org/ojs/index.php/rhs Research in Health Science Vol. 5, No. 4, 2020 27 Published by SCHOLINK INC.
Most previous studies have investigated the effects of SES markers such as household income (Kimberly , poverty (Kimberly , parental educational attainment (Assari, 2020;Assari, Boyce, Akhlaghipour, Bazargan, & Caldwell, 2020;, and neighborhood income (Jenkins et al., 2020;Parker et al., 2017;Willis et al., 2014)on children's brain function. However, all these SES indicators are objective measures and do not reflect the perception of the relative social standing of the family. Subjective family SES may reflect aspects of SES that are not reflected by education, employment, income, and poverty status. According to one study, subjective family SES increased individuals' vulnerability to stress for racial and ethnic minority families (Assari, Preiser, Lankarani, & Caldwell, 2018).
There is a need to test the additive and multiplicative effects of race, ethnicity, and family SES on brain development. As race, ethnicity, and family SES have close overlap (Kaufman, Cooper, & McGee, 1997), family SES may explain why race and ethnicity impact brain development. In this view, it is family SES, not race or ethnicity, which is the true driver of inequalities in children's brain development . At the same time, high family SES may have different effects on children's brain development across racial and ethnic groups (Kimberly . This is supported by a growing body of research on Minorities' Diminished Returns (MDRs) (Assari, 2017;, a phenomenon of weaker effects of SES on various health outcomes, including brain development of Black and Latino than non-Latino White children in terms of attention (Assari, Boyce, & Bazargan, 2020a), impulse control (Assari, Caldwell, & Mincy, 2018), inhibitory control (Assari, 2020), depression (Assari & Caldwell, 2018), suicidality , anxiety (Assari, Caldwell, & Zimmerman,

The ABCD Study Design and Setting
This secondary data analysis had a cross-sectional methodological design. We used the existing data from the ABCD study (Alcohol Research: Current Reviews Editorial, 2018;Casey et al., 2018;Karcher et al., 2019;Lisdahl et al., 2018;Luciana et al., 2018). This cross-sectional study used wave 1 data from the ABCD study. ABCD is a national brain imaging study of children's brain structure and function (Alcohol Research: Current Reviews Editorial, 2018;Auchter et al., 2018).

Ethics
The ethics review board approved the ABCD study of the University of California in San Diego. While youth signed assent, parents signed consent (Auchter et al., 2018). Given the de-identified nature of the data, our secondary analysis was determined to be non-human subject research by the Charles R Drew University of Medicine and Science ethics review board.

Samples and Sampling
The ABCD study participants were drawn from schools in various cities across various US states. The subject recruitment was mainly conducted through local school systems. Schools were selected based on characteristics such as race, ethnicity, SES, sex, and urbanicity (Garavan et al., 2018). This paper's analytical sample was 9390 non-Latino or Latino White or Black children who were between 9 and 10 years old. Inclusion in this analysis was limited to 9-10 years old children who had complete data on race, ethnicity, subjective family SES, and hippocampus volume.

fMRI and Image Acquisition
T1 weighted structural MRI images were taken using General Electric 750, 3 tesla (T) Siemens Prisma, and Phillips multi-channel coiled scanners. All MRI scanners were capable of multiband echo-planar imaging (EPI) acquisitions (Casey et al., 2018). A localizer was implemented at the beginning of the scan, followed by the T1 weighted structural image acquisition. Structural T1 weighted scan sequences were optimized for cortical / subcortical segmentation using a magnetization-prepared rapid acquisition gradient-echo.

Variables
The outcomes were right and left hippocampus size. We operationalized this variable as a continuous measure. We selected hippocampus volume as the outcome because adversities, poverty, economic Subjective Family SES. The independent variable was subjective family SES, measured by the following seven items: "In the past 12 months, has there been a time when you and your immediate family experienced any of the following": 1) "Needed food but could not afford to buy it or could not afford to go out to get it?", 2) "Were without telephone service because you could not afford it?" 3) "Did not pay the full amount of the rent or mortgage because you could not afford it?", 4) "Were evicted from your home for not paying the rent or mortgage?", 5) "Had services turned off by the gas or electric company, or the oil company would not deliver oil because payments were not made?", 6) "Had someone who needed to see a doctor or go to the hospital but did not go because you could not afford it?" and 7) "Had someone who needed a dentist but could not go because you could not afford it?" Responses were either 0 or 1. We calculated an average score with a potential range between 0 and 1-a score of 1 indicating highest possible subjective family SES. Our variable was a continuous variable (Assari, Preiser, et al., 2018 Age. Parents were asked to report children's age. Age was a dichotomous measure in years (9 vs. 10).
Sex. A dichotomous variable, sex was coded as male = 1 and female = 0.
Ethnicity. Parents were asked if they were from a Latino background. Ethnicity was coded as Latino = 1 and = 0.

Data Analysis
We started our data analysis by describing our study variables, using mean (SD) and frequency tables (%).
Then we calculated Pearson correlation to explore bivariate associations between our study variables. To perform multivariable analyses, two regression models were run. The first model did not include race/ethnicity by SES interactions. Model 2 did include such interactions. In the first step, we ruled out collinearity between study variables. We also tested the distribution of our outcome variables and error terms (residuals). The predictor variable was subjective family SES. The outcome was hippocampus volume. Confounders included race, ethnicity, sex, age, and parental marital status. Moderators were race and ethnicity. Regression coefficient, standard errors (SEs), confidence intervals (CIs), t-values, and p-values were reported. A p-value of less than 0.05 was significant. To perform our data analysis, we used SPSS.

Descriptives
This secondary analysis included 9390 9-10 years old children. From all of the participants, 23.5% were Black and 17.1% were Latino.     Table 3 shows the results of regression models in the total sample with right hippocampus volume as the outcome. Subjective family SES was positively associated with right hippocampus volume, net of confounders. This effect was significantly larger for Whites than Black families, documented by a significant interaction between race and subjective family SES on right hippocampus volume. Although the interaction between ethnicity and subjective SES was also in the same direction, it only approached significance but was not statistically significant. Outcome: Right Hippocampus Volume (mm3); SES: Socioeconomic Status  and reduced hippocampus connectivity with many brain regions and structures. The study also showed a link between family SES (poverty) and reduced functional brain connectivity between several brain regions and the hippocampus. The study also showed that the brain connectivity between the hippocampus and other brain regions might mediate the effect of childhood SES (poverty) on children's depression.
These parenting behaviors have a salient role in childhood brain development (Assari & Bazargan, 2019 poverty, and scarcity of resources which can interfere with the healthy development of youth's brains. We should re-emphasize that we see race and ethnicity as social not biological determinants of hippocampus volume. In our study, race and ethnicity are proxies of racism, poverty, SES, differential access to resources, and unequal treatment by society. This view is different from biological frameworks that conceptualize race and ethnicity as innate, unchangeable biological markers (Herrnstein & Murray, 2010).
We found that race, ethnicity, and subjective family SES have additive and multiplicative effects. As a result, low SES Black children are at the highest risk. As small hippocampus volume is linked to problems in memory and learning, cognition, and motions, future research should test how societal and structural conditions impact emotional, cognitive, and behavioral outcomes through altered hippocampus function (Chozick, 1983;Montagrin, Saiote, & Schiller, 2018;Yavas, Gonzalez, & Fanselow, 2019). The results of such investigations may have useful implications for clinical practice and public health, as well as social and economic policy on how to reduce racial and ethnic inequalities in brain development. The results of such investigations may direct us to how we can promote brain health equity through enhancing social justice across diverse social, economic, racial, and ethnic groups.
Our study findings suggested a major risk for both high and low-SES Black and Latino children. For White and non-Latino children, low SES is a risk factor, and high SES is a protective factor. For Black and Latino children, however, both those from low SES or high SES backgrounds remain at a similarly high risk in terms of small hippocampus volume. The smaller protective effects of SES for Blacks and Latinos may be due to the aspects of race and ethnicity that are not due to SES. These maybe because of race-and ethnicity related stressors like racism and discrimination, segregation, and blocked opportunities. Racial discrimination, stress, trauma, and adversities have all been shown to impact the hippocampus of Blacks and Latinos across all SES levels (Moadab, Bliss-Moreau, Bauman, & Amaral, 2017;Thames et al., 2018;Tottenham & Sheridan, 2009). The same may hold for other brain regions as well (Clark, Miller, & Hegde, 2018). Race, ethnicity, and SES have multiplicative and complex effects as social determinants of hippocampus activity. Thus, programs and interventions should be in place to alleviate the risk and promote brain development of low SES Black children. Early childhood programs and after school programs schools have received attention as programs effective at promoting brain development. (Garces, Thomas, & Currie, 2002;Gershoff, Ansari, Purtell, & Sexton, 2016;Neville et al., 2013;Zigler & Valentine, 1979). Other social and economic policies should reduce the environmental and structural adversities in the lives of Black families, across the full SES spectrum.

Limitations
To list the main methodological limitations, first, we should refer to the cross-sectional design of this investigation. As a result of this study's cross-sectional design, no causal associations can be made between subjective family SES and hippocampus volume. Second, we only studied one SES indicator, and several other indicators, such as income, employment, and social class were not included. We also www.scholink.org/ojs/index.php/rhs Research in Health Science Vol. 5, No. 4, 2020 34 Published by SCHOLINK INC.
described differential effects of SES and did not investigate how adversities, stress, neighborhood factors, and social context could explain such differential returns. Future research should investigate how contextual and environmental factors such as racial and ethnic density, similarity index, toxin exposure, and density of poverty and crime cause the observed MDRs.

Conclusions
In summary, high subjective family SES correlates with larger right and left hippocampus volume in a national sample of American children. However, these effects are weaker for Black than White families.