Background
Life history theory argues that unpredictable and harsh conditions such as early life adversity tends to produce a fast life history strategy, characterized by early sexual maturation and less parenting of offspring. It remains unclear whether all forms of early life adversity are associated with accelerated reproductive strategy, and most previous studies predominantly focused on single form of reproductive strategy indicators.
Objective
To examine the associations between 2 distinct dimensions of early life adversity (ie, threat and deprivation) and reproductive strategies across global metrics.
Study Design
We used data from 9674 middle-aged women of the China Health and Retirement Longitudinal Study. The experiences of threat and deprivation were assessed using the Life History Survey Questionnaire in 2014. Reproductive strategy information was assessed via self-report from the follow-up of 2013, 2015 and 2018, including age at menarche, age at natural menopause, age at first birth, total number of children, and number of abortions. Multivariate linear regression analyses were performed to assess the associations between distinct dimensions of early life adversity and multiple reproductive strategy indicators, adjusting for age, Hukou location, family socioeconomic status in adulthood and body mass index.
Results
Of the 9674 women (mean [standard deviation] age at baseline, 55.89 [10.23] years), 4084 (42.20%) reported exposure to threat-related early life adversity and 7332 (75.79%) reported exposure to deprivation-related early life adversity. Early life adversity characterized by threat was associated with accelerated reproductive strategy. Compared with women who have no experiences of threat-related early life adversity, ≥3 threat-related early life adversity was associated with 3.7-month earlier age at menarche ( β =−0.31, 95% confidence interval, −0.53 to −0.08; P =.007), 8.6-month earlier age at natural menopause ( β =−0.72, 95% confidence interval, −1.29 to −0.15; P =.013), >1-year earlier age at first birth ( β =−1.14, 95% confidence interval, −1.58 to −0.71; P <.0001), and an increased total number of children ( β =0.25, 95% confidence interval, 0.10–0.41; P =.002). In contrast, experiences of deprivation were associated with delayed age at natural menopause ( β =.50, 95% confidence interval, 0.06–0.94; P =.025) and increased number of abortions ( β =.17, 95% confidence interval, 0.01–0.34, P =.037), in models adjusting for co-occurring threat exposures.
Conclusion
This study suggests that early life adversity characterized by threat was associated with accelerated reproductive strategy, whereas deprivation was associated with slower reproduction strategy. Future research should clarify the biological pathways between different dimensions of early life adversity and reproductive strategies and further determine whether accelerated reproduction is an adaptive response to early life adversity in humans.
Introduction
Life history theory posits that individuals will select different allocation strategies according to the environment and life events experienced when resources are limited, trade-off physical growth and reproduction, so that resources and energy may be optimally allocated. Unpredictable and harsh conditions such as early life adversity (ELA) tend to produce fast life history strategy among women, characterized by earlier age at sexual maturation, earlier age at first birth, earlier age at menopause, and increased number of offspring.
Sexual maturity indicates the transition of energy expenditure from somatic growth to reproductive effort. Previous studies reported the relationships among various types of ELA and earlier age at menarche and menopause, including sexual abuse, maternal and paternal overprotection, family dynamics (eg, parental divorce) and parental mental illness. However, little evidence is available on the associations of ELA with indicators of reproductive success (fitness), for example, number of offspring and abortions. Research indicates that more ELA were associated with earlier age at first birth, and childhood experiences of physical and sexual abuse were individually associated with increased risk of miscarriage. In general, most of the research till date, focused on single form of reproduction strategy indicators; therefore, they were limited in their ability to capture the general picture of reproductive timing and reproductive success.
Why was this study conducted?
Most studies regarding the relationships between early life adversity (ELA) and reproductive strategies focused on single forms of reproductive indicators, and it remains unclear whether all forms of adversity are associated with accelerated reproductive strategy.
Key findings
Threat-related ELA was associated with accelerated reproductive strategy. Compared with women with no experiences of threat, ≥3 threat exposures were associated with a 3.7-month earlier age at menarche, 8.6-month earlier age at natural menopause, >1-year earlier age at first birth, and an increased total number of children. In contrast, experiences of deprivation were associated with delayed age at natural menopause and increased number of abortions, in models adjusting for co-occurring threat exposures.
What does this add to what is known?
A specific dimension of ELA, that is, threat but not deprivation, was associated with accelerated reproductive strategy.
It is important to acknowledge that these trends of ELA and faster life history are not universal. , Life history-related traits seem to covary along a dimension of slow vs fast life history, reflecting the different trade-offs that individuals face in different environmental contexts. The evolutionary framework of life history theory predicts that the diversity of reproductive strategies in nature is shaped by environmental cues such as the resource scarcity (ie, deprivation) and extrinsic morbidity and mortality (ie, threat). , In addition, recent conceptual models have emphasized the importance of distinguishing between threat-related ELA experiences characterized by environmental harshness (ie, abuse and violent exposure) and deprivation-related ELA experiences characterized by the lack of expected environmental inputs (ie, physical neglect, emotional neglect, and food insecurity). Findings from our studies and others , , suggested that threat-related ELA was related to advanced age at pubertal onset, whereas no similar results were observed for deprivation-related ELA. Thus, the present study aims to identify and distinguish specific associations between 2 ELA dimensions with indicators of reproductive timing, for example, age at menarche, menopause, and first birth, in addition to reproductive success, for example, number of offspring and abortions, in middle-aged women from the China Health and Retirement Longitudinal Study (CHARLS).
Materials and Methods
Study sample
This study used data from the CHARLS, which is a high quality nationally representative longitudinal survey of middle-aged residents in China, including assessments of social, economic, and health status. The baseline national wave of CHARLS was conducted in 2011, 3 follow-up surveys were carried out in 2013 (wave 2), 2015 (wave 3) and 2018 (wave 4), and physical measurements were conducted in every 2 follow-up cycles. Further details about the CHARLS have been previously documented.
In this study, we used data from the baseline (2011) and 3 follow-up (2013, 2015 and 2018) of CHARLS. Our analytical sample was restricted to those women aged 40 and older at baseline and had information about ELA (n=9674; mean [standard deviation] age at baseline, 55.89 [10.23] years).
Early life adversity exposure
Multiple ELA experiences characterized by threat and deprivation were assessed using the Life History Survey Questionnaire in 2014. Participants were asked whether they experienced the specified adversities before the age of 17 or not (detailed description has been provided in the Supplemental Methods ). Threat-related ELA included unsafe community dwelling, peer bullying, female guardian physical abuse, male guardian physical abuse, being beaten by a sibling and parental conflict; deprivation included biological mother absent, biological father absent, food scarcity, poor family economic conditions, and loneliness.
We created threat- and deprivation-related ELA composites by summing the total number of threat and deprivation experiences respectively. Based on the distribution of ELA ( Table 1 ), we categorized the number of cumulative childhood threat and deprivation adversities into 4 groups (0 adversity, 1 adversity, 2 adversities, ≥3 adversities) for analysis.
| Characteristic | Values | Valid (n) |
|---|---|---|
| Demographics | ||
| Age at baseline, mean (SD), y | 55.89 (10.23) | 9674 |
| BMI, mean (SD), kg/m 2 | 23.96 (4.14) | 6055 |
| Hukou location, % | 8587 | |
| City | 16.6 | 1428 |
| Between urban and rural | 6.3 | 544 |
| Rural | 76.6 | 6579 |
| Special zone | 0.4 | 36 |
| Education level, % | 8592 | |
| Illiterate | 34.9 | 2996 |
| Less than primary school | 22.2 | 1907 |
| Primary school | 18.7 | 1606 |
| Middle school | 16.5 | 1415 |
| No less than high school | 7.8 | 668 |
| Early life adversity exposure | ||
| Threat exposure composite, mean (SD) | 0.83 (1.11) | 8578 |
| Unsafe community dwelling, % | 9.4 | 892 |
| Peer bullying, % | 10.9 | 1049 |
| Female guardian physical abuse, % | 21.0 | 1968 |
| Male guardian physical abuse, % | 10.5 | 964 |
| Sibling violence, % | 6.9 | 664 |
| Parental conflict, % | 23.8 | 2111 |
| No threat events, % | 52.4 | 4494 |
| 1 threat event, % | 26.0 | 2233 |
| 2 threat events, % | 12.4 | 1067 |
| >3 threat events, % | 9.1 | 784 |
| Deprivation exposure composite, mean (SD) | 1.31 (1.02) | 9547 |
| Biological mother absent, % | 7.0 | 674 |
| Biological father absent, % | 8.9 | 860 |
| Food scarcity, % | 65.1 | 6268 |
| Poor economic conditions of family, % | 37.7 | 3641 |
| Loneliness, % | 16.9 | 1145 |
| No deprivation events, % | 23.2 | 2215 |
| 1 deprivation event, % | 37.3 | 3560 |
| 2 deprivation events, % | 28.4 | 2710 |
| >3 deprivation events, % | 11.1 | 1062 |
| Reproductive strategy indicators | ||
| Age at menarche, mean (SD), y | 16.02 (2.08) | 6575 |
| Age at natural menopause, mean (SD), y | 49.05 (4.52) | 5984 |
| Age at first birth, mean (SD), y | 23.48 (3.46) | 4058 |
| Total number of children, mean (SD) | 2.78 (1.49) | 5133 |
| Number of abortions, mean (SD) | 0.35 (0.70) | 1067 |
Reproductive strategy
Reproductive strategy outcomes were assessed from the follow-up of 2013, 2015 and 2018 via participant’s recall and mainly included age at menarche (AAM), age at natural menopause (AANM), age at first birth, total number of children, and number of abortions (including spontaneous abortions, still birth, and induced abortions). The AAM, AANM, and age at first birth were collected at whole-year increments (detailed description has been provided in the Supplemental Methods ).
Covariates
Models were adjusted for age, Hukou location (1=city, 2=between urban and rural, 3=rural, 4=special zone), family socioeconomic status in adulthood and body mass index (BMI). Hukou is a system of household registration used in mainland China. The representative index for family socioeconomic status in adulthood refers to education level (1=illiterate, 2=less than primary school, 3=primary school, 4=middle school, 5=no less than high school) and a total household annual expenditure per capita, in this study. BMI was calculated as weight (in kilograms) divided by the square of height (in meters). Height and weight were measured in 2015 and recorded in the physical examination questionnaire. Given that BMI can considerably affect the reproductive timing and reproductive success, we controlled it as a covariate in the models to eliminate confounding bias (detailed description has been provided in the Supplemental Methods ).
Statistical analysis
Data were analyzed using Statistical Product and Service Solutions, version 21.0 (IBM Corporation, Chicago, IL). The t test was used to compare the differences in reproductive outcomes between individuals with or without experiences of ELA. We used multivariate linear regression to estimate associations between reproductive strategy indicators as continuous variables and distinct dimensions of ELA as categorical variables, adjusting for baseline age, Hukou location, family socioeconomic status in adulthood, and BMI. Given the high co-occurrence of threat and deprivation adversities, , we estimated a model that included both dimensions of ELA, to evaluate the unique associations of each ELA dimension with reproductive strategies. Furthermore, we used all types of ELA exposure as independent variables in the multiple linear regression models to calculate tolerance and variance inflation factor (VIF), with a tolerance <0.1, and VIF >10 considered indicative of multicollinearity. The tolerance was >0.6, and VIF was <10 ( Supplemental Table ) for the ELA variables, indicating no collinearity among the independent variables. The results were considered statistically significant when P value was <.05.
Results
General characteristics
Sample characteristics are presented in Table 1 . A total of 9674 women were included in the study, the mean age at baseline was 55.89 (±10.23) years. The mean AAM was 16.02 (±2.08) years, the mean AANM was 49.05 (±4.52) years, the mean age at first birth was 23.48 (±3.46) years, the mean number of children was 2.78 (±1.49), and the mean number of abortions was 0.35 (±0.70).
Nearly half of the women (47.60%) experienced at least 1 form of threat-related ELA, mainly family conflict (23.80%) and female guardian physical abuse (21.00%), and 784 (9.14%) women reported 3 or more threat-related ELA.
More than three-fourths of women (76.80%) reported at least 1 form of deprivation-related ELA exposure, with food scarcity (65.09%) in childhood, accounting as the predominant ELA, and 11.12% reported 3 or more deprivation exposures.
The distribution of female reproductive strategy indicators in the population experienced threat-related or deprivation-related early life adversity
The distribution of reproductive strategy indicators in threat-related ELA populations is presented in Table 2 . Compared with women with no threat exposure, those with any threat-related ELA had an earlier age at first birth (all P values <.05). Women who experienced peer bullying or parental conflict had an earlier AAM ( P =.001 and P <.0001, respectively) and a decreased total number of children ( P <.0001 and P =.002, respectively) than those who experienced none. Furthermore, women exposed to unsafe neighbors, had an increased total number of children ( P <.0001) and abortions ( P =.006), women who experienced peer bullying had an increased number of abortions ( P =.002).
| Exposures | n (%) | Age at menarche mean (SD) | Age at natural menopause mean (SD) | Age at first birth mean (SD) | Total number of children mean (SD) | Number of abortions mean (SD) |
|---|---|---|---|---|---|---|
| Threat-related early life adversity | ||||||
| Unsafe community dwelling | 9467 | |||||
| Yes | 892 (9.4) | 15.88±2.10 | 48.76±4.76 | 23.13±3.72 | 3.11±1.66 | 0.51±0.80 |
| No | 8575 (90.6) | 16.03±2.07 | 49.06±4.49 | 23.51±3.41 | 2.74±1.46 | 0.32±0.68 |
| t value a | 1.731 | 1.443 | 2.119 b | 5.131 c | 2.758 b | |
| Peer bullying | 9647 | |||||
| Yes | 1049 (10.9) | 15.76±2.02 | 48.83±4.46 | 23.03±2.78 | 2.43±1.24 | 0.56±0.75 |
| No | 8598 (89.1) | 16.05±2.08 | 49.07±4.52 | 23.53±3.51 | 2.83±1.51 | 0.33±0.69 |
| t value | 3.251 d | 1.183 | 3.298 d | 7.001 c | 3.097 d | |
| Female guardian physical abuse | 9352 | |||||
| Yes | 1968 (21.0) | 15.96±2.10 | 48.86±4.52 | 23.01±3.30 | 2.75±1.51 | 0.43±0.89 |
| No | 7384 (79.0) | 16.03±2.07 | 49.08±4.50 | 23.63±3.46 | 2.75±1.46 | 0.33±0.67 |
| t value | 0.994 | 1.446 | 4.687 c | 0.075 | 1.164 | |
| Male guardian physical abuse | 9161 | |||||
| Yes | 964 (10.5) | 15.76±2.14 | 48.83±4.61 | 22.55±2.91 | 2.72±1.44 | 0.38±0.73 |
| No | 8197 (89.5) | 16.04±2.06 | 49.05±4.50 | 23.56±3.43 | 2.78±1.49 | 0.34±0.69 |
| t value | 3.311 c | 1.138 | 6.792 c | 1.190 | 0.835 | |
| Sibling violence | 9602 | |||||
| Yes | 664 (6.9) | 16.00±2.11 | 48.93±4.43 | 22.94±3.21 | 2.68±1.35 | 0.38±0.73 |
| No | 8938 (93.1) | 16.03±2.07 | 49.05±4.53 | 23.53±3.47 | 2.79±1.50 | 0.35±0.70 |
| t value | 0.295 | 0.478 | 3.099 d | 1.471 | 0.351 | |
| Parental conflict | 8888 | |||||
| Yes | 2111 (23.8) | 15.79±2.05 | 49.00±4.23 | 23.08±3.15 | 2.62±1.44 | 0.39±0.74 |
| No | 6777 (76.2) | 16.08±2.08 | 49.01±4.54 | 23.58±3.40 | 2.78±1.47 | 0.34±0.70 |
| t value | 4.512 c | 0.016 | 4.028 c | 3.138 d | 0.893 |
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