Decoding Nature and Nurture: Insights from Twin Studies

(Posted on Saturday, April 20, 2024)

KEY POINTS

• Twin study using brain imaging reveals genetic influence on cognition, with less impact on emotion processing.
• One notable finding suggests genetics play a role in distinguishing between disgust and fear.
• Understanding genetic and environmental interactions can inform mental health interventions.

The nature versus nurture conundrum is an eternal debate. A recent study of 175 identical and 88 fraternal adult twins explores some of the questions of how genes and the environment determine the fundamental aspects of the emotional and rational life of humans.

The study, led by Haeme Park at Neuroscience Research Australia and recently published in the Human Brain Mapping journal, used advanced brain imaging techniques to investigate cognitive and emotional processes. By examining data from twins, the researchers sought to unravel the relative influence of genes and the environment on brain function.

What Are Twin Studies?

Using structural equation modeling, it is possible to break down the total variability observed in a specific trait, like conscious emotion recognition or sustained attention, into different components that contribute to this variability. These components include genetic factors, shared environmental effects, and individual environmental effects. By understanding how much of the variance in a trait is due to genetics (heritability) versus environmental factors, researchers can gain insights into the underlying mechanisms influencing human behavior.

Twin studies involve comparing data from identical and fraternal twins to understand the role of genetics and environment on various traits. Identical twins share close to all of their genetic material, while fraternal twins only share about 50 percent, allowing for a comparison that highlights genetic influences. Because twins often grow up in the same place, there is also less variance in environmental factors.

The recent study by Park used data from the large TWIN-E cohort study, which includes 1,669 healthy Australian twin adults split almost evenly between identical and fraternal and male and female twin pairs. The goal of the ongoing TWIN-E study is to identify biomarkers that influence emotional brain health over time.

The researchers collected extensive data, including online assessments, electroencephalograms, functional magnetic resonance imaging (fMRI), and cognitive tasks. The fMRI data was obtained from a subset of 263 participants which were then included in Park’s study.

Cognitive and Emotional Tasks

Previous studies have determined that genetics play a significant role in the structural development of different regions of the brain, but few have looked at genetics and brain function using brain imaging while participants actively complete a task.

For the Park study, the twins completed five tasks while having their brains scanned using functional magnetic resonance imaging. Two tasks measured their emotional responses: a nonconscious processing of emotional faces task and a conscious processing of emotional faces task. The participants were shown standardized faces depicting anger, fear, sadness, disgust, happiness, or neutral expressions. For the nonconscious version, the emotional faces were shown for only ten milliseconds before being masked by a neutral face so that there would not be a conscious processing of the emotion. For the conscious version, the emotional face was presented for 500 milliseconds. The participants were asked at the end how many different emotions they observed for each task.

The other three tasks measured cognition: a working memory and sustained attention task, a response inhibition task, and a selective attention and novelty processing task.

The N-back test measured working memory and sustained attention by showing a letter on a screen for 200 milliseconds and asking participants to remember which letters were yellow.

The Go-NoGo task measured response inhibition and involved participants pressing on a green “go” stimulus but ignoring the red “NoGo” stimulus.

The Oddball task measured selective attention and novelty processing by asking participants to respond to audible tones presented at 1000 Hertz and ignoring the tones presented at 50 Hertz.

While participants worked on the tasks, the functional magnetic resonance imaging would light up, revealing which parts of the brain were activated. The researchers then measured the brain activation and compared them across participants.

Study Results

In order to quantify the associations of heritability and brain activity, the researchers used two different methods: a multivariate independent component analysis (ICA) approach and a univariate brain region-of-interest (ROI) approach.

Independent component analysis is a statistical analysis that involves separating data into independent components that represent different sources of information. Researchers can detect local functional connectivity networks within the brain and identify distinct patterns and structures within the data. The univariate region of interest approach allows researchers to focus on specific brain regions known to be involved in cognitive and emotional functions. This method involves analyzing the activity of these predefined brain regions to assess their heritability.

For the working memory, sustained attention, nonconscious processing of positive and negative emotional faces, and selective attention tasks, the participants’ brain function all showed a small to moderate genetic influence, while conscious processing of emotion and response inhibition showed no evidence of heritability. Overall, the functional networks related to executive functions showed the most prominent evidence of genetic influence.

The independent component analysis results showed that the heritability of brain function depended on the particular task. For subconscious emotion recognition, the brain network involving the superior temporal gyrus and insula showed a significant genetic influence when individuals were exposed to nonconscious disgust compared to neutral stimuli (26 percent) and nonconscious fear compared to happy stimuli (23 percent). For the working memory networks, including the fronto-parietal region and the inferior parietal lobule, a significant heritability estimate was found (27 percent). The sustained attention networks, including the superior temporal and precentral gyri, insula, pre- and post-central gyri, and the inferior parietal lobule, showed significant heritability (33 percent). Novelty processing networks had significant heritability in the superior and middle temporal gyri (33 percent) and the frontoparietal-temporal network (32 percent).

The brain region of interest approach had varying results. The ventral striatum showed 20 percent heritability for conscious facial emotion stimuli. The bilateral amygdala revealed a significant heritability contribution (right: 33 percent, left: 34 percent) elicited by nonconscious facial emotion stimuli. The selective attention and novelty processing task showed a significant contribution of heritability in the medial superior prefrontal cortex (29 percent). The working memory, sustained attention, and response inhibition tasks showed no significant contribution of heritability in the brain regions of interest.

One notable finding is that the results suggest genetics play a role in distinguishing between disgust and fear more so than positive emotions. The researchers state that this may be due to an evolutionary adaptation, as identifying threats is key to survival. In general, however, they speculate that environmental factors have a greater influence on the perception of emotional expressions since “the intentional (conscious) and accurate perception of others’ emotional expressions within a particular environmental context is a paramount skill for successful social interactions.” Because social expectations vary so widely across cultures, it follows that the environment and external influences play a greater role in shaping social and emotional interactions compared to genetics.

Future Directions

The study is one of the first to analyze the shared genetic and environmental correlations across heritable brain networks/regions across multiple tasks. The researchers used advanced technology and research methods to investigate the extent to which brain function elicited by executive function and emotion processing may be heritable.

The results are interesting, yet they do not provide definitive answers to the complex nature versus nurture debate. Twin studies can provide interesting new information and allow researchers to unravel genetic mysteries, but there are limitations to using twin models, including assumptions of equal environments and random mating. While uncommon, it is also possible for twins to have different biological fathers and share only 25 percent of their DNA. These limitations in twin research methods keep us from arriving at definitive conclusions regarding the influence of genetics on behavior.

Researchers continue to seek answers that may provide groundbreaking revelations. A recent twin study published in JAMA Psychiatry demonstrated the significant impact of the environment on mental health outcomes, studying twins who had adverse childhood experiences. Longitudinal studies tracking brain development over time could provide more insights into how genetic and environmental factors interact to influence cognitive and emotional processes. Additionally, advances in imaging technology and computational methods offer exciting opportunities to explore the neural mechanisms underlying genetic influences on brain function.

Understanding the genetic basis of cognitive and emotional processes could also have various practical implications, informing mental health treatment and intervention. Insights into the genetic underpinnings of emotional processing could inform therapeutic strategies for conditions such as anxiety and depression. By recognizing the role of both genetics and the environment in shaping brain function, clinicians can tailor interventions to meet the specific needs of each patient.

The study represents a significant step forward in our understanding of the genetic and environmental influences on brain function. By uncovering the complex relationship between genes, brain networks, and cognitive processes, the research opens new avenues for personalized approaches to mental health care and intervention. As we continue to unravel the mysteries of the human brain, studies like this provide valuable insights into the influence of biology on human behavior.

Read the article online on Psychology Today.