Imagine for a moment Mark Rutte in kindergarten. He was probably just a regular four-year-old boy, playing with his friends. But what did this play look like? And were there any signs, at that early age, that Rutte would ultimately emerge as the longest-serving Prime Minister of the Netherlands? Most probably yes, say Yvonne van den Berg and Toon Cillessen. The two developmental psychologists are the driving force behind the Nijmegen Longitudinal Study (NLS), a large-scale social science study that has been running for 25 years, since 1998. The study is concerned with the question of how children develop socio-emotionally in the course of their life: in relation to their parents, their peers, and other people who are important to them.
In longitudinal studies, the same subjects are studied repeatedly over a longer period of time. Usually just a few years, but in this case already a quarter of a century. The Nijmegen study involves more than 100 participants, who have been followed since they were 15 months old. “Those babies are now young adults with houses and jobs,” says Van den Berg. “It won't be long before some of them have children of their own.”
A kind of diary
Every few years, the researchers look the participants up. They study their upbringing, friendships, and social status. What is their relationship with their parents like? Are they popular? Are they bullied? How do they behave in groups? The study also looks at the interaction with the subjects’ personality, and biological aspects, such as hormones and the brain. The advantage of this kind of long-term study, says Cillessen, is that you can make links between events and situations that are years apart in time. “You can also retrospectively ask participants about their childhood, but our images of the past are often coloured: sometimes they're too rosy, other times too pessimistic. The data from longitudinal studies function as a kind of diary. We can really read back what a given situation was like at the time, and what factors played a role then. Such data are much more reliable.”
First thousand days
Originally, the researchers did not intend to follow the Nijmegen children for so long. The study was set up in 1998 by Professor of Developmental Psychology Marianne Riksen-Walraven. She wrote a two-year project plan: by studying young children at age 15 and 28 months, she hoped to find out how their development was affected by their parents’ behaviour and background. “She was spot on in intuiting that there was something important there,” says Van den Berg.
Riksen-Walraven is considered a pioneer in research into early childhood development. This is a field that is given more attention these days: books on upbringing emphasise how important a child's first thousand days are for the rest of their life. “But at the time, there was less interest in it,” emphasises Van den Berg.
Valuable data set
The fact that the Nijmegen study started out on such a small scale also had a practical reason. Cillessen: “It is nearly impossible in science to get funding for a study that lasts more than a couple of years. It does happen, but these are real exceptions. That is why longitudinal studies are designed pragmatically. We always look just a few years ahead. If the study is going well, and the results continue to be promising, we look for funding to prolong the project.” Around 2000, when Riksen-Walraven and her PhD candidates looked at the results of their study, they saw even more clearly the great value of the data set they had gathered. The 129 participants came from all layers of society, and the families had very different perspectives on life. This is often difficult to achieve in scientific research, which mostly tends to attract highly-educated parents. The researchers were curious to find out whether the children's early experiences could also be linked to their later development. This led to a plan to continue to follow the participants for a longer period of time.
When Riksen-Walraven retired in 2014, Professor Toon Cillessen replaced her. Later on, Yvonne van den Berg and brain researcher Karin Roelofs also joined the project team. Of the original 129 participants, 109 are still faithfully taking part in the study. The team has by now performed 14 ‘measuring rounds’. These rounds are different every time, says Van den Berg. “We use the instruments and methods that best suit a given life stage. At every stage, we ask ourselves: what is the most reliable method for measuring a research question, in view of the children's age and skills?” Take for example a theme like attachment, she says. “If you want to study how well a 15-month-old toddler is attached, it doesn't make sense to ask them questions. You have to observe them and run experiments. You could for example ask the mother to leave for a moment, or have a stranger walk in, and see how the child responds. With five- or six-year-olds, it can be very informative to ask them to make a drawing. Only when children are verbally and cognitively strong enough can you ask them direct questions in an interview or via a questionnaire.”
The power of relationships
The NLS is a broad study, but the research questions are always about the impact of relationships on the children's development. “We know that positive relationships are good for people,” says Cillessen. “Children who have a positive relationship with their parents, teachers, siblings, and classmates, do better in life, not only socially, but in all kinds of areas. They are even physically healthier, and have better jobs. The question is: Why is that? And conversely: What risk do children run who do not have such relationships?”
For example, the way in which parents communicate with their one- or two-year-old turns out to impact how popular and well-liked these children are later on. “One of our studies looks at how successful fathers and mothers are in picking up their toddler's signals,” explains Van den Berg. “It turns out that based on this, you can predict how five-year-olds will cope with stressful situations. The better the parents picked up their toddler's signals, the more resilient the child was later on. And also: the more popular they were with their peers.” But, she adds, it is not all so black-and-white. “If we observed a more negative relationship between a parent and a 15-month-old, that child would have more difficulties controlling its impulses later on. But strangely enough, this also correlated with them being more popular with their peers in primary school. They stepped more easily into the foreground, took charge – in short, they were cooler. This shows that a ‘negative’ experience in early childhood can also lead to something ‘positive’.”
Incidentally, popularity is not always a rosy predictor of future health, adds Cillessen. “Being popular can also cause stress. And some popular pupils are bullies. So being popular is not necessarily the same as being liked all the time, especially not in secondary school.”
Bullying and stress
The Nijmegen researchers also discovered that children who are bullied are wired differently when it comes to the biological stress system. “Such children display a much stronger stress response on regular school days,” says Van den Berg. “This means that they react much more strongly on such days.” And this is not the only difference, adds Cillessen: “We also studied the interaction between negative experiences in early childhood – for example being bullied, having a difficult relationship with your parents, or few friends – and brain maturation. What we found is that the two are indeed related. Such experiences have a negative effect on brain development.”
Van den Berg: “It continues to amaze me how powerful and important children's relationship with their primary caretakers is in the first years of life. Somehow, you think: these are such small babies, how aware could they be of what is going on? But these early relationships with the father or the mother have such a huge impact. Even looking at the participants now, you can see how accurately some of the things that happened when they were 15 months old predicted their current experiences. Just imagine, we're talking nearly 25 years later.”
Covid-19 pandemic
In deciding when to invite participants for a new measurement round, the researchers mostly look at the life stage they are in. “We study people's primary social relationships in every life cycle,” explains Cillessen. “At first, these were the relationships with parents or primary caretakers. Later on, we included childcare workers, teachers, friends, and a larger group of peers. Around age nine, for example, friends are very important. And of course, we wanted to study the transition to secondary school, at age 12 to 13. Now that the participants are adults, life partners – and at some point their own family – are becoming increasingly important.”
Some events evade prediction. When the COVID-19 pandemic broke out in March 2020, the team quickly designed a new project plan. Van den Berg: “Since we'd already spent years mapping all our participants’ relationships, we could determine that the relationship with the mother in particular determined how stressful a lockdown would be for a person. The better the relationship with their mother, the less difficult the pandemic was for the participants – even as adults.”
The researchers also think about what parents, teachers, and other educators can do to improve the children's relationships, for example in school. Van den Berg has a project called Stoeltjesdans (Musical Chairs), which helps teachers gain more insight into what is happening in the classroom between pupils. All pupils complete a short questionnaire: Who is popular? Who is helpful? Who gossips? “The results give a clear idea of the social dynamics in the classroom. Based on the test results, the Stoeltjesdans software program can also suggest the most suitable seating plan for a class, based on insights from research.”
Birthday cards
To what extent is a study like the NLS unique in the world? The researchers say that there are some comparable studies, but they are few and far in between. Van den Berg: “There is for example a large-scale European longitudinal study, which has been following 10,000 participants for years. But because of the large number of participants, it has much less depth and nuance. Observations of the kind that we do are practically not feasible with thousands of participants. It would cost too much time.”
In any case, the NLS is completely dependent on the participants’ enthusiasm, emphasises Van den Berg. She and her colleagues therefore devote a lot of time and effort to foster this relationship. The participants are sent a card on their birthday, they get an occasional phone call from the researchers, and they receive a small gift every time they take part in a study. “You have to understand: we come into their lives from early childhood on, at home and at school. We invite their friends to join in our study. And we ask them endless questions, about very personal things. The least we can do, is get to know them a little. I’m genuinely surprised at how committed our participants are. And without participants, no study. So if half of them drop out, we’ll have to stop.”
Detective skills
The advantage of doing research in the Netherlands, the researchers say, is that our society is relatively stable. Cillessen: “People tend to relocate only within the country. And the Netherlands is well organised, so it's easy to locate the participants. Our schools have good databases, and if you ask people to forward you their new address, they usually do. In the US, these things are much harder to organise. People tend to relocate over longer distances, for example to a different state. In this way, you can lose them.”
Did the NLS researchers also lose some participants over the years? Yes, says Van den Berg with a laugh, but we always managed to track them down in the end. “My detective skills have really improved – with thanks to social media. And we have at times shown up at someone's old address. Via the new inhabitants, we were able to get in touch after all. If people hadn't dropped out on purpose, they were usually quite willing to take part again.”
New generation
What does the future of the NLS look like? “As far as I'm concerned, we can keep going for a while,” says Cillessen. “We’re heading into an interesting new phase: the participants are developing their careers, graduating, and starting families of their own. This is something I'm really curious about. It would be interesting to also involve the next generation in the study. Will our participants treat their children in the same way their parents treated them?”
Van den Berg would like to learn more about how adults interact with their peers. “We know that peers are also important later on in life. But do people have for example one good friend, or a group of friends? I'm really curious about the impact of such relationships. What predictive value does this have for other areas of their life? Is it for example linked to how people deal with their life struggles?”
With this new phase, practical challenges are also emerging for the first time. Van den Berg: “So far, the children's lives have been quite similar: they lived at home with their parents, went to school, and later on to university. But now you see that their life paths are increasingly diverging. We can hardly say: if you could all make sure you have your first child by 2026, that would be really helpful. The same applies to marriages and illnesses – we can predict that these things will happen, but not when or to whom. This means that we already have to plan now what we want to study later on. As long as it makes sense from a scientific perspective, and the participants are enthusiastic, we will do what we can to continue with the study.”
Text: Susanne Geuze. This article was first published (in Dutch) in Radboud Magazine #73. Photo: Alexandr Podvalny via Unsplash