A study recently published in JAMA Psychiatry found that higher amounts of time spent in a low-income environment during childhood were associated with increased risk of schizophrenia after age 15. Below, researcher Christian Hakulinen, PhD, shares details of the study and its findings.
Q: What prompted your research on the connection between parental income in childhood and the risk of schizophrenia later in life?
A: We conducted this study as previous studies have showed mixed findings and there were only a few studies which used parental income as a marker of socioeconomic environment. We have also recently conducted a study on schizophrenia and later labor market outcomes and felt that it would be important to examine the role of early socioeconomic environment in later risk of schizophrenia.
Q: Please briefly describe your study and its findings.
A: In our register data-based study, we included all individuals born in Denmark from 1980 to 2000 who had two Danish-born parents and who were alive and living in Denmark at their 15th birthdays. Our study cohort included 1,051,033 persons who were followed from their 15th birthday until the first diagnosis of schizophrenia, date of death, emigration, or the end of 2016. Thus, individuals were followed for up to 22 years, up to their 37th birthday.
Information on schizophrenia diagnosis (ICD-10 code F20) was available from the Danish Psychiatric Central Research Register, which captures information on all inpatient psychiatric admissions since 1969 and all outpatient and emergency unit visits since 1995. Information on parental income was available from the Integrated Database for Labour Market Research and it was measured at birth and ages 5, 10 and 15. For the statistical analyses, parental income was grouped as quintiles.
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Our results showed that low parental income at any measurement point (birth and ages 5, 10, and 15) was associated with increased risk of schizophrenia. These associations were nonlinear, which means that the risk was considerably higher among those individuals who were at the lowest parental income quintiles. In terms of absolute risk, individuals from the lowest parental income had a 2.7% risk of being diagnosed with schizophrenia by age 37 years, whereas the same risk was 0.9% for individuals from the most affluent parental income quintile.
We also found that the more time individuals spent living in low parental income conditions during childhood, the higher the later risk of schizophrenia was. Importantly, we also found that no matter what the parental income level at birth was, upward parental income mobility between birth and age 15 was on average associated with a lower later risk of schizophrenia.
Q: Were any of the outcomes particularly surprising to the study team?
A: We were mainly surprised that our findings were so clear. In our additional analyses, we controlled for a number of important potential confounders or mediators, such as history of parental mental disorder, parental educational attainment, and the level of urbanicity at birth. With these adjustments, the hazard ratios were smaller, but nevertheless the risk remained.
Q: What are the possible real-world applications of these findings?
Although our findings might not be so straight forwardly transferred to clinical practice, they highlight the importance of early family socioeconomic environment in development of schizophrenia. Children who have lived their whole childhood in poverty could especially be targeted for preventive and early intervention measures.
Q: Do you and your co-investigators intend to expand upon this research? What further studies do you feel are needed?
A: In the future, we are planning to conduct some additional studies on the topic. I think it would be important to conduct multidisciplinary studies that would further examine in detail the mechanisms which explain the link between low childhood socioeconomic position and later schizophrenia risk.
Q: Is there anything else pertaining to your research and findings that you would like to add?
A: Our study highlights 2 important issues. First, the longer a person spent living in low-income conditions during childhood, the higher the subsequent risk of schizophrenia. Second, regardless of what was the parental income level at birth, upward income mobility between birth and age 15 was generally associated with a lower risk of developing schizophrenia.
Christian Hakulinen, PhD, is a post-doctoral researcher in Faculty of Medicine, University of Helsinki, Finland. In 2018, he was a visiting researcher at the National Centre for Register-based Research, University of Aarhus, Denmark. He earned his PhD in psychology in 2013 and his main topics of interest are social determinants and mental disorders; social relationships and health; and personality traits and health.
Hakulinen C, Webb RT, Pedersen CB, Agerbo E, Mok PLH. Association between parental income during childhood and risk of schizophrenia later in life. JAMA Psychiatry. 2019 October 23;[Epub ahead of print].