For decades, marriage rates in the United States have fallen, while the average age at which Americans have children has risen. Birth rates have declined alongside these trends. Health and Human Services Secretary Robert F. Kennedy Jr. has called the drop a “national security threat.” Pro-natalist voices within the Trump administration’s Make America Great Again movement argue that low fertility could threaten society itself.
In October, Kennedy warned at a White House briefing that the nation’s fertility rate is too low to maintain population stability. In 2023, the total fertility rate dropped to a historic low and continued sliding in 2024, reaching fewer than 1.6 live births per woman of childbearing age. This is well below the replacement rate of 2.1, the level needed to sustain the population from generation to generation.
While some women are choosing to have fewer or no children, many face fertility challenges. To address this, President Trump announced plans to partner with a drugmaker to offer discounted fertility medications and expand employer-provided fertility benefits. However, public discussion of treatments targeting male infertility has been limited, despite male factors contributing to roughly half of all infertility cases.
Male fertility is influenced by age, health, and weight, which affect sperm count and testosterone levels—both reportedly declining. Kennedy has sounded repeated alarms about these trends, including a claim in October that “today, the average teenager in this country has 50 percent of the sperm count, 50 percent of the testosterone as a 65-year-old man.”
The science is more nuanced. Researchers have debated whether sperm counts are truly falling. The “sperm count decline hypothesis” suggests that global sperm counts are dropping, with low counts indicating poorer overall health. A 2017 meta-analysis covering North America, Europe, Australia, and New Zealand found a 52 percent decline in sperm counts between 1973 and 2011. A 2022 follow-up confirmed similar declines across more countries. Rossella Cannarella, a clinician at Italy’s University of Catania, said these findings align with her research and clinical experience, linking declines to pollution and metabolic disorders such as obesity.
Endocrine-disrupting chemicals, found in plastics, food packaging, and pesticides, may also contribute to male infertility. These substances interfere with hormones like testosterone and thyroid, affecting sperm production. Kennedy highlighted endocrine disruptors at the White House briefing as a potential factor in falling fertility rates. Germar Pinggera, a researcher at Innsbruck Medical University, added that pollution, poor diet, and an imbalanced gut microbiome can also reduce sperm quality.
Yet, some studies suggest sperm counts may be stable among men without fertility challenges. Data on sperm quality remain inconclusive, with trends varying by region. Reevaluations of earlier studies also point to inconsistencies in measurement methods, and experts note that sperm count alone is not a definitive measure of fertility. Male fertility depends on sperm count, shape, motility, testosterone, age, and interactions with female reproductive biology.
Testosterone, essential for sperm production, peaks in adolescence and early adulthood before gradually declining. A 2021 study found a 25 percent decline in testosterone among young adult males between 1999 and 2016, with 20 percent showing clinical deficiency. Experts caution, however, that shifts in measurement techniques may exaggerate reported declines.
Overall, the role of male biology in falling birth rates remains unclear. Experts point to broader social trends as major factors. At a Harvard panel in August, Margaret McConnell, an associate professor of global health, noted that fertility declines largely reflect delayed childbearing. Leslie Root, a researcher at the University of Colorado Boulder, added that the U.S. population is still growing and experiences a natural increase in births over deaths.
Delays in childbearing often reflect informed reproductive choices, influenced by career goals, financial considerations, and technologies such as IVF and egg-freezing. These social and economic factors shape family size preferences and reproductive timing more than biological trends alone.
Despite alarmist rhetoric about declining sperm counts, much about U.S. fertility remains uncertain, spanning both scientific and social dimensions. Understanding birth rate trends requires examining male biology alongside the broader context of reproductive decision-making in society.
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