In his role as therapist, the Swiss psychologist and sibling researcher Jürg Frick sees the many diverse roles that brothers and sisters can play in our lives. The most influential factors are gender and age difference, but upbringing also plays a major role.
As part of early research into sibling relationships in the 1920s, the Austrian psychologist Alfred Adler focused initially on sibling constellations. The assumption was: firstborn children are more responsible, middle children are more cooperative and the babies of the family are more willing to take risks. But is that really true? 
Personality psychologist Julia Rohrer from the University of Leipzig has analyzed large data sets from long-term studies for the first time. Her research shows: the position within the sibling constellation has less influence on someone’s character than previously thought. One aspect stands out, however: studies show that women who grew up with younger brothers earn less than those with younger sisters. It’s hoped that further research can shed more light on the reasons for this.
Current medical research even enables babies to be born in order to save their sibling’s life. Take Jamie Whitaker from the UK, for example: A test-tube baby, he was created to be a perfect tissue match to help his brother Charlie, who had been born with a rare blood disorder. Jamie’s embryo was a 98 per cent genetic match with his brother Charlie, so Jamie could donate the stem cells needed for a life-saving transplant. They’re both healthy adults now. But what impact does selecting suitable embryos to create savior siblings have on the sibling relationship? To date, this practice is ethically contested and even banned in many nations.
Studies with identical twins are also producing increasingly significant results. The hope is that we’ll be able to answer the urgent medical questions of our era: What influences the incidence of obesity, cardiovascular disease and Long COVID? Are genetic factors decisive - or are external factors more important?