In this large population‐based study, either a beneficial or no effect of advanced parenthood on child problem behavior was observed

Parental Age and Offspring Childhood Mental Health: A Multi‐Cohort, Population‐Based Investigation. Maria A.J. Zondervan-Zwijnenburg et al. Child Development, July 31 2019, https://doi.org/10.1111/cdev.13267

Abstract: To examine the contributions of maternal and paternal age on offspring externalizing and internalizing problems, this study analyzed problem behaviors at age 10–12 years from four Dutch population-based cohorts (N = 32,892) by a multiple informant design. Bayesian evidence synthesis was used to combine results across cohorts with 50% of the data analyzed for discovery and 50% for confirmation. There was evidence of a robust negative linear relation between parental age and externalizing problems as reported by parents. In teacher‐reports, this relation was largely explained by parental socio‐economic status. Parental age had limited to no association with internalizing problems. Thus, in this large population‐based study, either a beneficial or no effect of advanced parenthood on child problem behavior was observed.

Since 1995, the mean maternal age at first birth has increased at a rate of .10 years per year in Organisation for Economic Co‐operation and Development countries, and in 2017 exceeded 30 years in the vast majority of these countries (OECD, 2017). Only in Mexico was the mean age of women at childbirth lower than 28 years, and only in eight countries was it between 28 and 30 years of age. Women's reproductive years generally range from about 15 to 45 years (Te Velde, 2002). Within this wide age range some periods are generally considered more suitable to have children than others, but which parental reproductive ages are optimal for offspring physical and mental health has been a matter of debate ever since individuals have engaged in active birth control. Whereas having children at an advanced age was quite common historically, when families tended to be larger (e.g., Desjardins, Bideau, & Brunet, 1994), the current trend to delay childbearing has given rise to public health concerns.

Concerns Regarding Delayed Childbearing

Concerns regarding delayed childbearing are understandable, as a large number of research reports highlight that increased maternal age at childbirth is associated with several adverse consequences, ranging from physical problems, such as increased Body Mass Index (BMI), blood pressure and height (Carslake, Tynelius, Van den Berg, Davey Smith, & Rasmussen, 2017) to psychiatric conditions, such as autism (Lee & McGrath, 2015; Sandin et al., 2012), bipolar disorder (Menezes, et al., 2010), symptoms of depression, anxiety, and stress (Tearne et al., 2016), and poor social functioning (Weiser et al., 2008). More recently, increased paternal age at birth has also been associated with adverse child outcomes, such as stillbirth and cleft palate (see Nybo Andersen & Urhoj, 2017, for a review). In over 40 million live births between 2007 and 2016, having an older father increased the risk of low birthweight, Apgar score, and premature birth (Khandwala et al., 2018). A study of the Danish population, which included 2.8 million persons, found that older fathers are at risk of having offspring with intellectual disabilities, autism spectrum disorders, and schizophrenia (McGrath et al., 2014; see De Kluiver, Buizer‐Voskamp, Dolan, & Boomsma, 2017 for a review).

Several, not mutually exclusive, mechanisms have been proposed to explain the increased physical and mental health risks in offspring of older parents. First, age‐related deterioration of the functioning of women's reproductive organs, such as DNA damage in germ cells, and worse quality of oocytes and placenta, can increase the risk of obstetric and perinatal complications (Myrskylä & Fenelon, 2012). Second, male germline cells undergo cell replication cycles repeatedly during aging, with de novo point mutations accumulating over time (e.g., Jónsson et al., 2017) and the number of de novo mutations in the newborn increasing with higher age of the father at the time of conception (Francioli et al., 2015; Kong et al., 2012). Although weaker than with paternal age, de novo mutations in offspring correlate with maternal age as well (Goldmann et al., 2018; Wong et al., 2016). Third, genomic regions in the male germline may become less methylated with increasing age (Jenkins, Aston, Pflueger, Cairns, & Carrell, 2014) and alter the expression of health‐related genes. Fourth, age effects can be due to selection, with older parents differing from younger ones in characteristics that are relevant for developmental outcomes in their offspring, such as poor social skills. The influence of selection effects can be exacerbated by assortative mating (Gratten et al., 2016). Fifth, being the child of older parents carries the risk of having to cope with parental frailty or losing a parent at a relatively young age (Myrskylä & Fenelon, 2012), and the stress evoked by these experiences may trigger health problems. Most of these mechanisms involve consequences of biological aging. Parenthood at an advanced age is disadvantageous from a biological perspective; except for very young, physiologically immature mothers, younger parents are in a better physical condition.

Possible Benefits of Delayed Childbearing

Whereas the effects of older parental age on children's physical health and psychiatric disorders tend to be predominantly negative, the effects of older parental age on mental health problems with a stronger psychosocial component, such as externalizing and internalizing problems, tend to be more inconsistent. An indication that the negative consequences of high parental age may stretch beyond clinical diagnosis is provided by Tearne et al. (2015, 2016), who found that high maternal age predicted symptoms of depression, anxiety, and stress in daughters, and by Janecka, Haworth, et al. (2017) who reported a negative association between advanced paternal age and social development. In contrast, in several population‐based studies, offspring of older parents, particularly of older mothers, perform better at school and work, score higher on intelligence tests, report better health and higher well‐being, use fewer drugs, and have fewer behavioral and emotional problems than offspring of younger parents (e.g., Carslake et al., 2017; McGrath et al., 2014; Myrskylä & Fenelon, 2012; Myrskylä, Barclay & Goisis, 2017; Orlebeke, Knol, Boomsma, & Verhulst, 1998; Tearne et al., 2015).

While the biology of aging seems to put older parents in an unfavorable position with regard to their offspring's physical and mental health, these contradictory effects of parental age on offspring mental health outcomes might be explained by a psychosocial perspective. Being a child of older parents can have substantial benefits (Lawlor, Mortensen, & Andersen, 2011), as older parents not only are often in a better socioeconomic position than young parents (Bray, Gunnell, & Davey Smith, 2006), thereby providing a more favorable environment for children, they also have greater life experience. Furthermore, older parents display more hardiness (McMahon, Gibson, Allen, & Saunders, 2007) and tend to have less substance use and fewer mental health problems (Kiernan, 1997), hence score higher on parenting factors that promote health and development (Janecka, Rijsdijk, et al., 2017; Kiernan, 1997). In part, positive associations of advanced parental age could be related to selection effects. In young people, substance abuse and related externalizing problems go together with earlier sexual activity (Crockett, Bingham, Chopak, & Vicary, 1996), which increases the probability that intergenerational transmission of externalizing problems occurs at an early parental age (Bailey, Hill, Oesterle, & Hawkins, 2009). Like age‐related parental characteristics that may have negative effects on offspring outcomes, the influence of such selection effects can be exacerbated by assortative mating (Gratten et al., 2016).

In sum, whereas advanced parenthood, particularly advanced paternal age, has primarily been associated with physical health and neurodevelopmental outcomes, such as autism and schizophrenia, advanced parenthood, particularly advanced maternal age, rather seems to predict mental health problems with a stronger psychosocial component, such as externalizing problems. Although it seems plausible that parental age interferes with subclinical problems and traits underlying these conditions, comprehensive evidence from population‐based cohorts is scarce and inconsistent, and more empirical evidence is desirable. Moreover, prior population‐based studies that used continuous measures of mental health problems usually focused on cognitive or behavioral problems (e.g., Carslake et al., 2017; Orlebeke et al., 1998) and, with a few exceptions that require replication in other cohorts (Janecka, Haworth, et al., 2017; Tearne et al., 2015, 2016), rarely included internalizing problems. A final reason to extend the research conducted thus far with this study is the wide variety of populations, designs, and outcomes used, which makes it hard to distinguish between substantive variation in association patterns and sample‐specific artefacts. In short, there is a need for studies that investigate both maternal and paternal age effects on continuously assessed core dimensions of offspring mental health (including internalizing problems) and that use robust analytical methods are suitable for the investigation of increased risk for both young and old parenthood.