Your Tier 1 audience is shrinking, and the demographic shift will reshape affiliate marketing GEOs.

Global fertility has fallen below replacement level for the first time in recorded history, and Tier 1 markets like the US, France, South Korea, and Taiwan are at the center of it. 

For affiliate marketing, that means traffic costs in these GEOs are heading toward a permanently higher baseline, not a temporary spike, while some currently secondary markets have a limited window to become the next growth GEOs.

Most performance marketers track traffic costs, conversion rates, and audience behavior on a weekly basis. Very few track the underlying demographic trends that determine whether those audiences will exist at all in ten years. That is about to become a more expensive oversight.

What the research says.

A study published in August 2026, Terra Incognita: The Economics of a Shrinking World, by economists Jesús Fernández-Villaverde of the University of Pennsylvania and Patrick Norrick of Northwestern University, confirms that global fertility has fallen below replacement level for the first time in recorded history. Fertility is declining in 219 of 236 countries studied, with no sign of the trend reversing anywhere. World population is expected to peak around nine billion near 2056, then begin to decline.

A separate 2024 Lancet study projected that 155 of 204 countries would be below replacement fertility by 2050, and 198 by 2100. That same study projects sub-Saharan Africa will account for every second child born on the planet by 2100, and that the share of global births in low-income countries will grow from 18% in 2021 to 35% by 2100. The two studies use different methodologies but point in the same direction.

What surprised even the authors of the 2026 study is how much faster and wider the decline is spreading than standard demographic models predicted. The assumption was that low fertility would stay concentrated in wealthy nations. The data shows it's moving faster in low- and middle-income countries than expected.

Where Tier 1 is heading.

The numbers for the wealthiest markets are already here.

The US fertility rate is projected to hit 1.58 in 2026, the lowest on record, and keep falling toward 1.53 by the mid-2030s. France recorded more deaths than births in 2025 for the first time since World War II. South Korea and Taiwan are both below 1.0, meaning women there are having fewer than one child on average. Europe has been below replacement level since the 1970s and has only stayed stable through immigration.

For affiliate marketing, this translates directly into traffic costs. When advertiser competition rises seasonally or due to market activity, costs tend to level off. When competition rises because the audience is physically shrinking, there's no leveling off. The US, France, South Korea, and Taiwan aren't looking at temporary CPM spikes. They're looking at new baselines that will only move upward.

Where growth is still happening.

Africa remains the only region with fertility above replacement level. The Lancet projects sub-Saharan Africa alone will account for every second child born on the planet by 2100. Parts of LATAM, the Middle East, and Southeast Asia still have young and growing populations. These GEOs currently look like secondary markets, places you move into once Tier 1 runs out of headroom.

The research changes that framing. If fertility decline is spreading faster than expected into lower income countries, the window for entering these markets while audiences are young and traffic is cheap isn't unlimited. Affiliates building partnerships and testing these GEOs now are accumulating an advantage that won't be available to late movers.

The verticals this reshapes

Verticals tied directly to birth rates, products for young families, children, and parenting, are watching their natural base in Tier 1 narrow for years ahead. The audience isn't disappearing overnight, but the structural trend is clear and moving in one direction.

Meanwhile, verticals tied to aging populations, health, elderly care, retirement planning, financial services, and insurance are getting a structural tailwind in the same countries where fertility is falling. It's a demographic shift, not a market timing call.

The planning horizon most performance marketers are ignoring

The affiliate marketing industry is built around quarterly thinking: campaign cycles, payout periods, seasonal peaks. That cadence works for optimizing what exists today. It doesn't work for positioning ahead of a shift that plays out over decades.

A world population peak around 2056, followed by continued decline, means the GEO mix that performs today won't be the same mix in ten to fifteen years. The question isn't whether this will affect affiliate marketing. It's whether you're factoring it into decisions now, or waiting until the effect is impossible to ignore.

One caveat worth keeping in mind: growth regions today aren't guaranteed to stay growth regions tomorrow. The pace of fertility decline has surprised researchers who have spent careers studying it. Building toward LATAM or Southeast Asia makes sense, just without assuming these regions are permanently insulated from the same scenario.

Frequently asked questions

Why are Tier 1 traffic costs rising in affiliate marketing? Fertility in Tier 1 markets like the US, France, South Korea, and Taiwan has fallen well below replacement level, which shrinks the audience pool over time. Unlike seasonal competition spikes, this kind of demographic-driven cost increase doesn't level off.

Which GEOs still have growth potential for affiliate marketing? Africa is currently the only region with fertility above replacement level. Parts of LATAM, the Middle East, and Southeast Asia still have young, growing populations, though the window for cheap traffic in these regions is narrowing faster than expected.

Which affiliate verticals benefit from global fertility decline? Verticals tied to aging populations, including health, elderly care, retirement planning, and insurance, are seeing a structural tailwind in markets where fertility is falling. Verticals tied to young families and children are seeing their natural audience narrow in the same markets.

At Offerpump, we work with affiliates and traffic sources across multiple GEOs and verticals. If you want to talk through what this means for your traffic strategy, send us a message.