Why Measuring Millionaire Migration Is So Difficult

Policymakers, politicians, journalists, and investors increasingly want to know exactly how many people are moving across borders — who is leaving, where they are going, and in what numbers. That appetite is understandable: migration carries real implications for labor markets, public services, tax revenues, and national competitiveness. The demand for precision intensifies when the question concerns specific types of movers — whether asylum seekers, skilled workers, or high-net-worth individuals. Each cohort attracts its own political debates, data limitations, and policy pressures. The focus here is on one such group: the wealthy, whose migration is regularly discussed, cited in revenue forecasts, and debated in policy circles — yet remains exceptionally difficult to measure. Estimates of a single country’s outflows can vary by an order of magnitude depending on which definition of ‘millionaire’ is applied and what threshold is used to classify someone as having left. That divergence is structural, not a failure of reporting, and understanding it matters for interpreting any estimate, however carefully constructed.

The Migration Measurement Conundrum

Migration is the most difficult of the three components of demographic change to measure. Births and deaths are registered through administrative systems in virtually every country, producing near-complete, internationally comparable records. Migration has no equivalent infrastructure. Emigration poses a particular challenge: administrative systems capture arrivals far more reliably than departures, since people leave without formally deregistering, retain multiple addresses, and maintain ties that blur the boundary between residing somewhere and visiting it. As Willekens, Massey, Raymer, and Beauchemin observed in their assessment of global migration data,¹ the absence of reliable information on who leaves a country and when creates systemic uncertainty that frustrates both public debate and policy formation. The same constraint explains why estimates of global migration flows must be reconstructed through indirect statistical methods rather than read from any existing administrative record.²

Why the Wealthy Are Different

All of these challenges apply when it comes to high-net-worth individuals, and then others arise that are qualitatively distinct. Migration theory and the data infrastructure built around it were designed for another kind of mover: the labor migrant responding to wage differentials, employment opportunities, or conditions of insecurity at origin. Wealthy individuals relocate for different reasons. Their decisions are shaped by fiscal architecture, the stability of legal institutions, the availability of investor and residence programs, and the protection of assets across borders. In Tiebout’s³ terms, they are voting with their feet among competing jurisdictions — but the criteria are fiscal and regulatory rather than public-service provision, and the tools designed to count labor migration were never calibrated to track this population.

Definition, Data, and the Limits of What Exists

The definitional problems compound at every level. There is no agreed international threshold for a ‘high-net-worth individual’: definitions based on liquid investable assets, total net worth, or income produce substantially different populations, and any threshold is unstable as wealth fluctuates with asset valuations. Determining where someone ‘lives’ is equally contested. The globally mobile wealthy routinely hold residence rights across several jurisdictions at once, and fiscal domicile, habitual residence, and physical presence do not map onto a single administrative category.

This is not merely a conceptual issue. A detailed study of UK non-dom taxpayers found that a significant reform reducing net-of-tax rates produced a departure rate of under 5% among affected residents, and many of those who formally departed continued to spend most of their time in the country.⁴ Studies of US state tax changes reached similar conclusions: departure rates among high earners were modest and concentrated in specific circumstances, far below what revenue projections had assumed.⁵ Program applications, similarly, often measure demand for optionality rather than realized movement: second citizenships and residence rights are widely acquired as insurance against future instability, not as declarations of intent to relocate.⁶

The data gap is correspondingly wide. The Bloomberg Billionaires Index⁷ and Forbes’s Real-Time Billionaires ranking⁸ track the wealth of the world’s wealthiest individuals daily — yet neither provides systematic information on where those individuals actually live. If even the most closely watched segment of global wealth lacks reliable residence data, the challenge is far greater for the much larger population of high-net-worth individuals. A significant share of high-net-worth-individual wealth is held through offshore structures and trusts that are largely invisible to national statistical systems,⁹ further complicating efforts to identify the relevant population. There is no harmonized cross-border register of fiscal residence, and the methods used to reconstruct general migration flows have not yet been adapted or validated for this specific population.

A Different Approach — And What Comes Next

Given these constraints, the methodology underlying this year’s Henley Private Wealth Migration Report takes a different path. Rather than producing a precise count of movements that the data does not yet support, it focuses on what can be measured rigorously: the structural conditions that make a country more or less attractive to globally mobile wealth. Fiscal architecture, investor and residence and citizenship programs, institutional quality, geopolitical stability, capital mobility, and quality of life are assessed against publicly traceable data.

The Global Wealth Mobility Framework (GWMF) has been validated against Henley & Partners’ own enquiry and application pipeline — the most direct real-world signal on high-net-worth-individual mobility intentions currently available. That dataset carries a known limitation: it captures individuals who have already decided to seek professional advice about relocating, not the full population of potentially mobile wealthy individuals. Even with this selection bias acknowledged, the directional alignment is clear: countries with high Wealth Mobility Competitiveness Scores owing to their structural attractiveness consistently appear as rising destinations in the firm’s Q1 2026 enquiry data.

The ambition to produce credible estimates of wealth migration flows remains intact. The GWMF lays the groundwork for doing so. Structural conditions can be identified, measured, and tracked over time — and as the evidence base matures and is cross-referenced against proprietary enquiry data, administrative records, and emerging sources, it will provide the analytical foundation that any credible account of high-net-worth-individual inflows and outflows will require. The goal is to bring rigorous flow estimates back into reach. This framework is designed to build towards that.

References

¹ F. Willekens, D. Massey, J. Raymer, and C. Beauchemin (2016) ‘International Migration Under the Microscope’ Science 352(6288), 897–899 science.org/doi/10.1126/science.aaf6545 accessed 29 May 2026

² G. J. Abel and N. Sander (2014) ‘Quantifying Global International Migration Flows’ Science 343(6178), 1520–1522 science.org/doi/full/10.1126/science.1248676 accessed 29 May 2026

³ C. M. Tiebout (1956) ‘A Pure Theory of Local Expenditures’ Journal of Political Economy 64(5), 416–424 jstor.org/stable/1826343 accessed 29 May 2026

⁴ A. Advani, D. Burgherr, and A. Summers (2025) ‘Taxation and Migration by the Super-Rich’ CESifo Working Paper No. 11870 SSRN dx.doi.org/10.2139/ssrn.5254084 accessed 29 May 2026

⁵ C. Young, C. Varner, I.Z. Lurie, and R. Prisinzano (2016) ‘Millionaire Migration and Taxation of the Elite: Evidence from Administrative Data’ American Sociological Review 81(3), 421–446 doi.org/10.1177/0003122416639625 accessed 29 May 2026

⁶ A. Shachar (2009) The Birthright Lottery: Citizenship and Global Inequality Harvard University Press.

⁷ Bloomberg (n.d.) Bloomberg Billionaires Index bloomberg.com/billionaires accessed 29 May 2026

⁸ Forbes Magazine (n.d.) Forbes Real Time Billionaires List - The World’s Richest People forbes.com/real-time-billionaires accessed 29 May 2026

⁹ G. Zucman (2015) The Hidden Wealth of Nations: The Scourge of Tax Havens University of Chicago Press.