How does your family size compare?

In the US, the mean age at first birth for women has risen from 21.4 years in 1970 to 27.0 years in 2021 (CDC). In the UK, the mean age at first birth is 31 years (ONS 2021). Both figures reflect a long-term trend toward later parenthood driven by extended education, later marriage, and changing economic conditions.

Quite common. CDC data shows 26.4% of US women aged 30 to 34 have no children. This does not mean these women will remain childless; many will have children in their mid-to-late 30s or early 40s. By ages 40 to 44, the figure falls to around 14%. Having no children at 30 is well within normal variation for the current generation.

Research on this question is mixed and context-dependent. A large meta-analysis found no consistent relationship between number of children and life satisfaction across countries. In some high-income countries with good parental support systems, having children is associated with modest increases in wellbeing. In others with high costs and limited support, the reverse is observed. Parenthood satisfaction depends heavily on support systems, financial security, partnership quality, and whether the timing was chosen or not.

Pew Research Center 2024 data found that among US adults under 50 without children who do not plan to have them, 64% cited simply not wanting children as a reason. An additional subset cited environmental concerns, financial concerns, or health reasons. Among adults over 50 who are childless, 38% said there was a time they actively wanted children but it did not happen, showing that a significant portion of childlessness is circumstantial rather than chosen. The voluntary childlessness rate has been rising consistently across high-income countries, with particularly sharp increases among women with higher educational attainment.

Higher female educational attainment is one of the strongest predictors of smaller completed family size and later first birth. ONS and CDC data both show a clear inverse relationship between years of education and completed fertility. Women with postgraduate degrees have a completed fertility rate approximately 0.5 to 0.7 children lower than women who left education earlier. The effect is partly direct (extended education delays first birth, reducing available childbearing years) and partly mediated by career opportunities, income, and shifting life priorities. The relationship holds across most countries and ethnicities when controlling for religious affiliation.

Secondary infertility is the inability to conceive or carry a pregnancy after having already had one or more biological children. CDC estimates suggest it affects approximately 11% of couples who already have a child. It is often more isolating than primary infertility because affected individuals may feel they cannot claim a fertility struggle having already had children. Causes include age-related fertility decline, acquired conditions such as endometriosis or pelvic inflammatory disease, changed sperm parameters, and complications from previous deliveries. It explains part of the gap between desired and achieved family size in survey data.

CDC data shows that among US women who have completed their childbearing (ages 40 to 44), approximately 18% have three children and around 10% have four or more. ONS data for England and Wales shows similar proportions: three-child families represent about 17% and four-or-more around 8%. Both figures have declined substantially since the 1970s. Large families now cluster more strongly in religiously observant households and lower-income populations in both US and UK data.

Research shows a nuanced picture. Children born to older parents face modestly elevated risks of certain chromosomal conditions: Down syndrome risk rises from approximately 1 in 1,000 at maternal age 30 to around 1 in 100 at age 40. Paternal age over 40 is associated with slightly increased risk of autism spectrum disorder and some rare genetic mutations. However, children of older parents also benefit from greater financial stability, more established relationships, and more experienced parenting. Population-level studies show modest advantages in educational attainment for children of older parents once socioeconomic status is controlled, partially offsetting the biological risk increase.

The "spoiled only child" stereotype is not supported by modern research. Meta-analyses consistently show that only children perform at least as well as children with siblings on measures of intelligence, academic achievement, and psychological adjustment. Some studies show small advantages in achievement motivation and verbal ability for only children. Only children do show slightly lower scores on cooperativeness, but these differences are small and largely explained by parenting style rather than sibling presence per se. The historical negative reputation of only children in research literature has been substantially revised over the past two decades.

Both, and they are difficult to disentangle. Housing costs, childcare costs, and wage stagnation clearly contribute: surveys consistently show that people across high-income countries report wanting more children than they have, with financial constraints most commonly cited. But cultural factors operate independently: longitudinal data shows fertility declining even among high-income groups where financial constraints are minimal, suggesting a shift in how parenthood competes with other life goals. Research by Lesthaeghe on the Second Demographic Transition points to post-materialist value shifts, including prioritising self-realisation, career, and personal experience, as a separate mechanism running alongside economic constraints.

A large-scale meta-analysis by Margolis and Myrskyla found that the relationship between number of children and parental wellbeing is highly nonlinear and context-dependent. In countries with generous parental leave and subsidised childcare (Scandinavia, Germany), having one or two children is associated with modest wellbeing gains. In countries with high childcare costs and limited parental leave (US, UK), the wellbeing benefit is smaller or absent for more than one child. Most studies find that the transition from zero to one child has the largest impact on wellbeing (positive or negative depending on context), while subsequent children have diminishing returns or negative marginal effects on parental life satisfaction.