A major goal of the Sibling Interaction and Behavior Study (SIBS) is to understand how the relationships among family members evolve over time. Thanks to your participation in the study over the past 30 years, SIBS families have given us a unique opportunity to study families. Although there are many factors that affect family relationships, we have been especially interested in exploring the impact of adoption. 

Are adoptive families as cohesive as non-adoptive families? 

Most of the adoption research before SIBS only studied adopted children, but SIBS is the first study to study both adopted children as well as their adoptive parents. New results from the SIBS Study are beginning to reveal how adoption affects family cohesion, or how much family members stay connected with one another. The graph below illustrates how connectedness differs across age groups within adoptive and non-adoptive families.

Percent of individuals who have contact with a family member several times/week

As shown above, the frequency of contact in early adulthood is greater, but this contact decreases as family members grow older. However, differences between adoptive and non-adoptive families are very small, suggesting that adoptive families are just as connected as non-adoptive families.

In 1890, the Harvard professor William James - often called the Father of Psychology - declared that by the age of 30 our personality was “set like plaster, and will never soften again.” It was a gloomy thought. After all, personality contributes to many important life outcomes including the quality of our relationships, the risk of mental disorders, our occupational successes and even how long we might live. Fortunately, in this case the professor was wrong.  

Research over the past few decades - including research undertaken at the MCTFR - has shown that our personalities continue to change and adapt throughout our lives. The accompanying figure plots the average score for MCTFR participants at different ages for three broad personality traits: 

• Positive Emotionality (blue): The tendency to be positively and actively engaged, happy, and optimistic
• Negative Emotionality (red): The tendency to experience negative mood states like anxiety and hostility
• Self-Control (green): The tendency to deliberate and sacrifice immediate reward to attain long-term goals 

Although the level of Positive Emotionality remains relatively flat from mid-adolescence through midlife, as Professor James would expect, Self-Control increases with age while Negative Emotionality decreases. Psychologists call this pattern the “maturity principle” - as we age we tend to experience fewer turbulent moods and are more likely to think before we act.

Nonetheless, you may notice that there is an unexpected slight uptick in Negative Emotionality and a downturn in Positive Emotionality at age 42.  Is this a mere statistical blip or a real phenomenon, perhaps reflecting the stresses that accompany middle-age?  Continued follow-up of the MCTFR participants will allow us to answer this and other important questions about how our personalities evolve over time. 

An analysis of all twin studies in 2015 found 2,748 different studies with over 14 million twin pairs. That's a lot of twins! To compare, the first twin study from the MCTFR, the Minnesota Twin Family Study (MTFS) beginning in 1989, included 1881 twin pairs and is still ongoing.

Through adoption studies like SIBS, we can investigate the impact of genetics compared to shared environments. Biological siblings show us the impact of both genetics and common environments whereas adopted siblings show just the impact of a common environment. By comparing these two groups, we’ve found that personality is influenced more by genetics than by shared environments. 

While personality isn’t impacted by a common environment, other behaviors and beliefs are moderately impacted. In adolescence, this includes social values, substance use, mental health, and academics. However, in adulthood the influence of a shared environment seems to diminish and focus on social values and academics. 

In this talk, MCTFR Principal Investigator Matt McGue discusses the role our families have in our psychological development:

Self-reports of  screen time, sleep duration, and physical activity tend to be inaccurate. The ABCD Study uses the Effortless Assessment Research System (EARS) app and Fitbits to help collect objective measures of screen time, sleep, and physical activity. These measures provide support for relationships between increased screen time, sleep disruptions, and reduced physical activity.

Late-night screen time is common among adolescents. In a study of 791 adolescents using the EARS app to monitor screen time, 84% of adolescents typed on their phone’s keyboard between 10:00pm and 6:00am at least once. Also, nearly three quarters of nights recorded app usage between 10:00pm and 6:00am.

Does the time of day spent on your phone matter? It does! Every hour of smartphone usage during the day corresponds to 420 fewer steps, 3 fewer minutes of exercise, and up to 13 more sedentary minutes. During the night, every hour spent on a smartphone corresponds to 10 fewer minutes of sleep, falling asleep 22 minutes later, and waking up 15 minutes later. 

Citation
Alexander, J. D., Nguyen-Louie, T. T., Gupta, S., Cummins, K. M., & Wade, N. E. (2025). Adolescent smartphone use, sleep, and physical activity: Daily Associations between Sensor-Based Measures in the Adolescent Brain Cognitive Development (ABCD) Study. Psychiatry Research, 349, 116523. https://doi.org/10.1016/j.psychres.2025.116523