Association Between Parental Income During Childhood and Risk of Schizophrenia Later in Life

Abstract

Importance: Evidence linking parental socioeconomic position and offspring's schizophrenia risk has been inconsistent, and how risk is associated with parental socioeconomic mobility has not been investigated.

Objective: To elucidate the association between parental income level and income mobility during childhood and subsequent schizophrenia risk.

Design, setting, and participants: National cohort study of all persons born in Denmark from January 1, 1980, to December 31, 2000, who were followed up from their 15th birthday until schizophrenia diagnosis, emigration, death, or December 31, 2016, whichever came first. Data analyses were from March 2018 to June 2019.

Exposure: Parental income, measured at birth year and at child ages 5, 10, and 15 years.

Main outcomes and measures: Hazard ratios (HRs) for schizophrenia were estimated using Cox proportional hazard regression. Cumulative incidence values (absolute risks) were also calculated.

Results: The cohort included 1 051 033 participants, of whom 51.3% were male. Of the cohort members, 7544 (4124 [54.7%] male) were diagnosed with schizophrenia during 11.6 million person-years of follow-up. There was an inverse association between parental income level and subsequent schizophrenia risk, with children from lower income families having especially elevated risk. Estimates were attenuated, but risk gradients remained after adjustment for urbanization, parental mental disorders, parental educational levels, and number of changes in child-parent separation status. A dose-response association was observed with increasing amount of time spent in low-income conditions being linked with higher schizophrenia risk. Regardless of parental income level at birth, upward income mobility was associated with lower schizophrenia risk compared with downward mobility. For example, children who were born and remained in the lowest income quintile at age 15 years had a 4.12 (95% CI, 3.71-4.58) elevated risk compared with the reference group, those who were born in and remained in the most affluent quintile, but even a rise from the lowest income quintile at birth to second lowest at age 15 years appeared to lessen the risk elevation (HR, 2.80; 95% CI, 2.46-3.17). On the contrary, for those born in the most affluent quintile, downward income mobility between birth and age 15 years was associated with increased risks of developing schizophrenia.

Conclusions and relevance: This study's findings suggest that parental income level and income mobility during childhood may be linked with schizophrenia risk. Although both causation and selection mechanisms could be involved, enabling upward income mobility could influence schizophrenia incidence at the population level.

Figure 1.. Hazard Ratios (HRs) of Schizophrenia Risk According to Parental Income Quintiles at Age 15 Years

A, In the basic adjustment, HRs are adjusted for sex, birth year, and calendar time. B, In the additional adjustment, HRs are adjusted for sex, birth year, calendar time, parental mental disorders, parental educational attainment level, degree of urbanicity at birth, and number of changes in child-parental separation status. Error bars indicate 95% CIs.

Figure 2.. Cumulative Incidence of Schizophrenia by Parental Income Quintiles (Qs) at Age 15 Years

The cumulative incidence percentage value measures the risk (probability) of developing schizophrenia before a given age. Quintile 1 is the lowest income quintile, and Q5 is the highest income quintile. The model is adjusted for sex (reference, male) and birth year (reference, 1980).

Figure 3.. Hazard Ratios (HRs) of Schizophrenia Risk by Cumulative Parental Income Scale During Childhood

The parental income scale represents relative parental income levels and their duration between birth and 15th birthday. The minimum possible score of 4 represents parental income being in the lowest level, quintile 1, across all 4 age points (ie, at birth, and at ages 5, 10, and 15 years). A maximum score of 20, the reference category for HR estimation (HR = 1), represents parental income being consistently in the highest level, quintile 5. A, In the basic adjustment, HRs are adjusted for sex, birth year, and calendar time. B, In the additional adjustment, HRs are adjusted for sex, birth year, calendar time, parental mental disorders, parental educational attainment level, degree of urbanicity at birth, and number of changes in child-parental separation status. Error bars indicate 95% CIs.

Figure 4.. Hazard Ratios (HRs) of Schizophrenia Risk According to Parental Income Mobility Between Birth and Age 15 Years

The reference group for HR estimation is parental income quintile 5 at both birth year and at age 15 years (HR = 1). A, In the basic adjustment, HRs are adjusted for sex, birth year, and calendar time. B, In the additional adjustment, HRs are adjusted for sex, birth year, calendar time, parental mental disorders, parental educational attainment level, degree of urbanicity at birth, and number of changes in child-parental separation status. Error bars indicate 95% CIs.

Figure 5.. Cumulative Incidence of Schizophrenia Associated With the Lowest (Quintile 1 [Q1]) and the Highest (Q5) Parental Income Levels in the Year of Birth

The cumulative incidence percentage value measures the risk (probability) of developing schizophrenia before a given age. Models are adjusted for sex (reference, male) and birth year (reference, 1980).