Brain and molecular mechanisms underlying the nonlinear association between close friendships, mental health, and cognition in children
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eLife assessment
The findings of this study yield important new insights into the relationship between the number of close friends and mental health, cognition, and brain structure. Due to the large sample sizes, the evidence is solid but would have been improved if both of the analyzed datasets contained more closely matched measures. This work advances our understanding of how the friendship network relates to young adolescents' mental well-being and cognitive functioning and their underlying neural mechanisms.
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Abstract
Close friendships are important for mental health and cognition in late childhood. However, whether the more close friends the better, and the underlying neurobiological mechanisms are unknown. Using the Adolescent Brain Cognitive Developmental study, we identified nonlinear associations between the number of close friends, mental health, cognition, and brain structure. Although few close friends were associated with poor mental health, low cognitive functions, and small areas of the social brain (e.g., the orbitofrontal cortex, the anterior cingulate cortex, the anterior insula, and the temporoparietal junction), increasing the number of close friends beyond a level (around 5) was no longer associated with better mental health and larger cortical areas, and was even related to lower cognition. In children having no more than five close friends, the cortical areas related to the number of close friends revealed correlations with the density of μ-opioid receptors and the expression of OPRM1 and OPRK1 genes, and could partly mediate the association between the number of close friends, attention-deficit/hyperactivity disorder (ADHD) symptoms, and crystalized intelligence. Longitudinal analyses showed that both too few and too many close friends at baseline were associated with more ADHD symptoms and lower crystalized intelligence 2 y later. Additionally, we found that friendship network size was nonlinearly associated with well-being and academic performance in an independent social network dataset of middle-school students. These findings challenge the traditional idea of ‘the more, the better,’ and provide insights into potential brain and molecular mechanisms.
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Author Response
Reviewer #1 (Public Review):
The goal of this study was to examine the nature of the relationship between a number of close friends and mental health, cognition and brain structure. In particular, the authors were interested in any potential non-linear relationships between a number of close friends and various measures (neurocognition, brain structure).
Strengths
The sample sizes are very large (total size > 23,000) across two datasets.
There are a wide range of measures in the ABCD dataset -- mental health, cognition and brain data.
There were two independent datasets and the results were broadly similar across datasets.
The longitudinal aspect (2-year follow up) to the data is also a strength, as is the use of cross-lagged panel models.
The use of the two-lines test -- formally testing a non-linear …
Author Response
Reviewer #1 (Public Review):
The goal of this study was to examine the nature of the relationship between a number of close friends and mental health, cognition and brain structure. In particular, the authors were interested in any potential non-linear relationships between a number of close friends and various measures (neurocognition, brain structure).
Strengths
The sample sizes are very large (total size > 23,000) across two datasets.
There are a wide range of measures in the ABCD dataset -- mental health, cognition and brain data.
There were two independent datasets and the results were broadly similar across datasets.
The longitudinal aspect (2-year follow up) to the data is also a strength, as is the use of cross-lagged panel models.
The use of the two-lines test -- formally testing a non-linear relationship among variables -- is a notable strength (many studies only test using a quadratic equation, which does not necessarily mean that any relationship is significantly non-linear).
We thank the reviewer for endorsing our effort to use two extensive and independent datasets, as well as various statistical methods, to examine the nonlinear association between the number of close friends and mental health, cognition and bran structure.
Weaknesses
The study is associational and causal relations cannot be determined (the authors' themselves are clear on this point).
We agree with the Reviewer’s comment that as an association study, no causal conclusion should be made in this study, which we have clearly stated as a limitation in the Discussion (see Lines 360-364, Page 18).
The measures in the two datasets were not identical, precluding a direct out-of-sample validation test.
We agree with the Reviewer that the social network dataset is not a direct out-of-sample validation as the measures of mental health and cognition are not identical due to the data limitation. To avoid repetition, we have answered this question in detail in Page 14 in the response letter.
The depth of the information about friend relationships in the ABCD study was limited. The number of close friends was recorded, but not the quality of those relationships.
We agree with the Reviewer that the measure of close friendship in the ABCD study is simple. It is essential to incorporate friendship quality in future studies. We have discussed this limitation in the manuscript.
“Third, the quality of close friendships was not considered in the ABCD study. However, reciprocal degree is an indirect measure of friendship quality, which was found to be linearly associated with well-being and nonlinearly related to the GPA in the social network dataset. It has been reported that the relationship between having more friends and fewer depressive symptoms in adolescence is mediated by a sense of belonging (Ueno, 2005). Although current findings on the relative importance of friendship quantity and quality are inconsistent (Bruine de Bruin et al., 2020; Platt et al., 2014), it is essential for future studies to incorporate measures of close friendship quality and to test the potential interaction between quantity and quality.” (Lines 372-380, Page 19)
To the extent that the authors were attempting to show relations among variables - and not causal associations - the authors have achieved their aims. An impact of these results lies in the link between 'Dunbar's number' of close relationships and neurocognitive measures, supporting the link between social relationships and brain and cognition in humans. The brain data in ABCD were very rich and notably allowed the authors to investigate neurotransmitter density. This is not a weakness of the study per se but it is notable that the effect sizes are quite small (although highly significant given the large sample sizes).
We agree with the Reviewer that the large sample size derives low P-values for even small effect sizes. Please refer to the detailed reply in Pages 2-3 in the response letter.
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eLife assessment
The findings of this study yield important new insights into the relationship between the number of close friends and mental health, cognition, and brain structure. Due to the large sample sizes, the evidence is solid but would have been improved if both of the analyzed datasets contained more closely matched measures. This work advances our understanding of how the friendship network relates to young adolescents' mental well-being and cognitive functioning and their underlying neural mechanisms.
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Reviewer #1 (Public Review):
The goal of this study was to examine the nature of the relationship between a number of close friends and mental health, cognition and brain structure. In particular, the authors were interested in any potential non-linear relationships between a number of close friends and various measures (neurocognition, brain structure).
Strengths
The sample sizes are very large (total size > 23,000) across two datasets.
There are a wide range of measures in the ABCD dataset -- mental health, cognition and brain data.
There were two independent datasets and the results were broadly similar across datasets.
The longitudinal aspect (2-year follow up) to the data is also a strength, as is the use of cross-lagged panel models.
The use of the two-lines test -- formally testing a non-linear relationship among variables -- is …Reviewer #1 (Public Review):
The goal of this study was to examine the nature of the relationship between a number of close friends and mental health, cognition and brain structure. In particular, the authors were interested in any potential non-linear relationships between a number of close friends and various measures (neurocognition, brain structure).
Strengths
The sample sizes are very large (total size > 23,000) across two datasets.
There are a wide range of measures in the ABCD dataset -- mental health, cognition and brain data.
There were two independent datasets and the results were broadly similar across datasets.
The longitudinal aspect (2-year follow up) to the data is also a strength, as is the use of cross-lagged panel models.
The use of the two-lines test -- formally testing a non-linear relationship among variables -- is a notable strength (many studies only test using a quadratic equation, which does not necessarily mean that any relationship is significantly non-linear).Weaknesses
The study is associational and causal relations cannot be determined (the authors' themselves are clear on this point).
The measures in the two datasets were not identical, precluding a direct out-of-sample validation test.
The depth of the information about friend relationships in the ABCD study was limited. The number of close friends was recorded, but not the quality of those relationships.To the extent that the authors were attempting to show relations among variables - and not causal associations - the authors have achieved their aims. An impact of these results lies in the link between 'Dunbar's number' of *close* relationships and neurocognitive measures, supporting the link between social relationships and brain and cognition in humans. The brain data in ABCD were very rich and notably allowed the authors to investigate neurotransmitter density. This is not a weakness of the study per se but it is notable that the effect sizes are quite small (although highly significant given the large sample sizes).
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Reviewer #2 (Public Review):
This is a novel and interesting study in which the authors aimed to gain a better understanding of whether there is an optimum number of close friends to gain good mental well-being/functioning and its underlying neural mechanisms. They thoroughly examined how the number of close friendships contributes to mental health, cognition, (social) brain structure, and neural molecular processes in adolescents. They conducted multiple analyses on two large datasets to answer their research question(s) and support the results with visually attractive figures. I believe this paper is of added value to the literature as the evidence presently robustly points to the optimum number of 5 close friends in relation to mental health and cognition and related neurobiological mechanisms. This greatly advances the knowledge in …
Reviewer #2 (Public Review):
This is a novel and interesting study in which the authors aimed to gain a better understanding of whether there is an optimum number of close friends to gain good mental well-being/functioning and its underlying neural mechanisms. They thoroughly examined how the number of close friendships contributes to mental health, cognition, (social) brain structure, and neural molecular processes in adolescents. They conducted multiple analyses on two large datasets to answer their research question(s) and support the results with visually attractive figures. I believe this paper is of added value to the literature as the evidence presently robustly points to the optimum number of 5 close friends in relation to mental health and cognition and related neurobiological mechanisms. This greatly advances the knowledge in the field of social and neurocognitive psychology.
The authors use a variety of measures to assess mental health, cognition, and neural mechanisms, which is a strength of the study. However, the theoretical background of these constructs should be elaborated on or unpacked to a greater extent in the introduction. Relatedly, the discussion could benefit from clearer main messages conveyed by individual paragraphs. It is currently hard to follow how the authors interpret their results in the context of existing literature.
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