We Trust What We Know
A consistent finding across social psychology and finance is that people favour people, companies, and assets that feel familiar or are associated with their in-group. This preference cannot be fully explained by genuine differences in quality or risk — it operates even when the familiar option is objectively inferior to an unfamiliar alternative.
Social Identity Theory and In-Group Bias
The theoretical foundation was laid by social psychologist Henri Tajfel at the University of Bristol and later Oxford, with colleague John Turner, through their development of Social Identity Theory (1979) and later Self-Categorisation Theory (1987). Tajfel and Turner showed, through the "minimal group paradigm" experiments, that people display in-group favouritism based on the most trivial distinctions — even random assignment to Group A or Group B created measurable preference for one's own group.
In-group bias in finance means we unconsciously favour companies, funds, or investment styles that are associated with our identity — whether professional, cultural, national, or social. A banker may be biased toward financial sector investments. A tech professional may systematically overweight technology stocks. These are not necessarily poor decisions, but the preference often precedes the analysis rather than following from it.
Home Bias in Investing
One of the most extensively documented financial manifestations of familiarity bias is home bias — the tendency for investors to hold a disproportionate share of their portfolios in domestic equities, despite the clear benefits of international diversification.
Economists Kenneth French and James Poterba documented the phenomenon comprehensively in a 1991 paper, finding that investors in the US, Japan, UK, France, and Germany each held approximately 90% or more of their equity portfolios in domestic stocks, despite those markets representing a much smaller fraction of global equity value.
The implication is significant: home bias concentrates risk in a single geography and market cycle, reducing the free diversification benefit available through international holdings. The rational basis for home bias (currency risk, information advantage in domestic markets) is insufficient to explain its actual magnitude.
The Familiarity Bias
A related concept was formalised by Gur Huberman in a 2001 paper titled "Familiarity Breeds Investment." Analysing data from the US Regional Bell Operating Companies (RBOCs) after the AT&T breakup, Huberman found that investors were dramatically more likely to hold shares in their own regional telephone company than in other equally accessible RBOCs. There was no rational informational or quality advantage — investors simply preferred what they knew.
Huberman's finding extended to employee stock ownership: workers hold substantially more of their own employer's stock than diversification principles would recommend. This concentrates personal financial risk precisely where employment income risk already exists — a compounding of vulnerabilities that is the opposite of rational risk management.
The Diversification Cost
Both home bias and familiarity bias impose a measurable cost in the form of foregone diversification. The investor who holds only domestic stocks, only familiar companies, or only investments associated with their professional background is taking on avoidable concentration risk.
The corrective is not to invest in unfamiliar assets for their own sake, but to apply explicit, evidence-based criteria to investment selection — and to treat the feeling of familiarity as a factor to scrutinise rather than a signal to trust.