Making New People

Kevin Lewis

January 21, 2024

Population and Welfare: The Greatest Good for the Greatest Number
Mohamad Adhami et al.
NBER Working Paper, December 2023 


Economic growth is typically measured in per capita terms. But social welfare should arguably include the number of people as well as their standard of living. We decompose social welfare growth -- measured in consumption-equivalent (CE) units -- into contributions from rising population and rising per capita consumption. Because of diminishing marginal utility from consumption, population growth is scaled up by a value-of-life factor that exceeds one and empirically averages nearly 3 across countries since 1960. Population increases are therefore a major contributor, and CE welfare growth around the world averages more than 6% per year since 1960 as opposed to 2% per year for consumption growth. Countries such as Mexico and South Africa rise sharply in the growth rankings, whereas China, Germany, and Japan plummet. These results are robust to incorporating time use and fertility decisions using data from the U.S., Mexico, the Netherlands, Japan, South Africa, and South Korea. Falling parental utility from having fewer kids is roughly offset by increases in the "quality" of kids associated with rising time investment per child.

The Nurture of Nature and the Nature of Nurture: How Genes and Investments Interact in the Formation of Skills
Mikkel Aagaard Houmark, Victor Ronda & Michael Rosholm
American Economic Review, forthcoming 


This paper studies the interplay between genetics and family investments in the process of skill formation. We model and estimate the joint evolution of skills and parental investments throughout early childhood. We document three genetic mechanisms: the direct effect of child genes on skills, the indirect effect of child genes via parental investments, and family genetic influences captured by parental genes. We show that genetic effects are dynamic, increase over time and operate via environmental channels. Our paper highlights the value of integrating biological and social perspectives into a single unified framework.

The Labor Market Effects of Pregnancy Accommodation Laws
Emily Battaglia & Jessica Brown
University of Delaware Working Paper, December 2023 


We estimate the effect of pregnancy accommodation laws on labor market outcomes for women of childbearing age. Such laws require "reasonable accommodations" for pregnant workers, like sitting down, lifting restrictions, and additional bathroom breaks. As a mandated benefit, these laws could impact employer hiring decisions. Using a triple differences design comparing women's and men's labor market outcomes throughout the staggered roll-out of thirteen pregnancy accommodation laws from 2013 to 2016, we find no overall impact on female employment and wages. We do find that women are more likely to choose occupations where physical abilities are important, suggesting increased accessibility of these occupations. But for subgroups more likely to be impacted -- those with less education, in more physically-intense occupations, and married without children -- we estimate modest declines in earnings and employment. That the burden falls on both suggests these women value the benefit but at less than it costs to provide.

Exploring Sex Differences in Trajectories of Pubertal Development and Mental Health Following Early Adversity
Tiffany Ho et al.
Psychoneuroendocrinology, March 2024


Despite evidence that early life adversity (ELA) affects mental health in adolescence, we know little about sex differences in how distinct dimensions of adversity affect development and their corresponding effects on mental health. In this three-wave longitudinal study, 209 participants (118 females; ages 9-13 years at baseline) provided objective (salivary hormones, BMI, age of menarche) and subjective (perceived gonadal and adrenal status) measures of puberty and physical development, and reported on levels of internalizing and externalizing symptoms at all timepoints. Participants also reported lifetime exposure to three distinct types of ELA: deprivation, threat, and unpredictability. Using generalized additive mixed models, we tested within each sex whether dimensions of adversity were associated with longitudinal changes in measures of pubertal and physical development, and whether these indices of development were associated with trajectories of internalizing and externalizing symptoms. In females, experiences of threat and unpredictability were significantly associated with earlier pubertal timing (e.g., age of menarche) whereas experiences of deprivation were associated with steeper increases in BMI; further, faster pubertal tempo (i.e., steeper increases in pubertal stage) was associated with increases in internalizing and externalizing symptoms. In males, however, ELA was not associated with any measures of pubertal or physical development or with symptoms. Together, our results suggest that adverse experiences during early life have sex-selective consequences for pubertal and physical maturation and mental health trajectories in ways that may elucidate why females are at higher risk for mental health difficulties during puberty, particularly following exposure to unpredictable and threatening experiences of adversity.

Is reproductive development adaptively calibrated to early experience? Evidence from a national sample of females
George Richardson et al.
Developmental Psychology, forthcoming 


Many developmental theories have not been sufficiently evaluated using designs that control for unobserved familial confounds. Our long-term goal is to determine the causal structure underlying associations between early environmental conditions and later psychosocial and health outcomes. Our overall objective in this study was to further evaluate predictions derived from applications of life history theory to female reproductive development, key among them that reproductive milestones translate early environmental risk into fertility, health, and behavioral outcomes. To this end, we used female data from the National Longitudinal Survey of Youth 1979 and structural equation modeling to conduct increasingly severe tests, beginning with covariate control and then progressing to sibling control and behavioral genetic designs. After adjusting for confounds varying between sets of siblings, we did not find evidence that age at menarche reflected components of early environment or that any focal outcomes reflected early fragmented family structure (birth to age nine). Although we detected no links between measured environment and individual differences in age at sexual debut, we did find that it reflected both shared and nonshared influences in our behavior genetic models. Interestingly, delayed sexual debut (into young adulthood) reflected identification of parents as the greatest influences and forecasted an array of fertility-related outcomes. Taken together, these findings challenge theories suggesting menarche timing is adaptively calibrated to early environment. They also highlight the need for more research using sibling control and related designs to examine the roles of environments in development.


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