Failure to Incorporate Behavioral Assumptions Accurately is one of the biggest sources of error in Law & Economics, Valuation, Forensic Evidence, and Regulatory Law. It happens when a model, expert, employer, or regulator assumes people are perfectly rational homo economicus, while real people act with bounded rationality, bounded willpower, and bounded self-interest.

Traditional law and economics assumes:

• Unbounded rationality - people process all information perfectly
• Unbounded willpower - they act on long-term interests
• Unbounded self-interest - they maximize only their own utility

Behavioral Law and Economics, pioneered by Jolls, Sunstein and Thaler, shows empirical evidence that people actually exhibit systematic biases: optimism bias, hindsight bias, loss aversion, status-quo bias, availability heuristic. When you fail to incorporate that, your legal or economic conclusion fails.

Why This Failure Matters Legally

"If most people living under a particular legal rule — say, a rule requiring a warning statement describing the risks of a product — will tend to assume, as a result of their unrealistic optimism, that the risks are much lower for them personally, then the legal rule will not have the effects that traditional economic theory would predict."

This creates 3 types of legal failure:

1. Expert Evidence Rejection: Opinion collapses because its factual/behavioral foundation is false.
2. Discrimination / Wrongful Dismissal: Employer makes stereotypical behavioral assumptions about capacity.
3. Regulatory / Antitrust Failure: Regulator assumes rational consumer switching, when consumers are locked by defaults and biases.

Case Laws Where Courts Punished This Failure

A. Employment & Discrimination - Stereotypical Behavioral Assumptions

1. Chief Constable of Norfolk v Lisa Coffey [2019] EWCA Civ 106
Employer refused police transfer assuming her mild hearing loss would deteriorate and she would go on restrictive duties. The Court of Appeal held this was direct discrimination by perception. Norfolk did not properly explore facts before making stereotypical assumptions about effects of her condition. Same principle reiterated in later commentary: employers should base decisions on abilities at that particular time, not assumptions about future deterioration.

2. Grujicic v Lineage Aust Trs Pty Ltd [2024] FWC 3526
Fair Work Commission found dismissal unfair because employer made incorrect assumptions about employee's capacity to fulfill inherent requirements, instead of relying on medical assessment.

3. Toulouse Court of Appeal, Airbus Appraisal Case - 21 September
Appraisal based partly on behavioural criteria held illegal. Court said requesting certain behaviours violated spirit of Article L.1222-3 French Employment Code.

B. Expert Evidence - Failure to Prove Behavioral / Factual Assumptions

This is the most litigated area.

4. Kingston v Head, Transport for Victoria; Jomaring v Head (No 2) [2024] VSC 231
Expert evidence rejected because it was based on assumptions that were unsupported by a reasonable interpretation of law and evidence. Rule: Assumptions given to experts must be supported by reasonable interpretation of law and evidence.

5. Beagle v Australian Capital Territory [2016] ACTSC 271
Expert had appropriate expertise but evidence unclear about source of facts and assumptions. Found unreliable.

6. Capitol Carpets Pty Ltd v Schwartz Family Co Pty Ltd [2016] NSWSC 1753
Expert made assumptions about nature and quality of carpet based on a packing slip for an order delivered 4 years later. Court held failure to prove factual assumptions which underpin an expert's report must at least be taken into account in assessing weight that can properly be accorded to opinion.

7. Harris v Johnston (2023, UK Clinical Negligence)
Neurosurgeon expert made factual assumptions about key matters without checking. Had to be recalled to witness box after erroneous assumptions demonstrated.

8. JBX v Frimley Health NHS Foundation Trust [2026] EWHC 2294
High Court rejected expert testimony where expert failed to consider full range of accommodation options and respective advantages.

C. Antitrust, Consumer & Constitutional Law - Behavioral Economics Overrides Rational Choice Model

9. United States v. Google LLC (2020) - US DOJ
Government alleged search dominance partly due to ability to manipulate consumer choice through default settings and behavioral biases. Core theory: default agreements constitute de facto exclusive dealing due to infrequency of users changing defaults, a classic status-quo bias.

10. FTC v. Facebook, Inc. (2021)
FTC alleged acquisitions of Instagram and WhatsApp motivated by desire to eliminate potential competition - a behavioral assumption about network effects and loss aversion in user switching.

11. Expressions Hair Design v. New York - US Supreme Court 2016 (cert granted)
First major effort to ground First Amendment law in behavioral economics. New York's no-surcharge law case. Argument relied on claim derived from behavioral economics: surcharge vs discount framing changes consumer behavior even if economically identical.

12. European Commission v. Google (2019) - Shopping case
Commission found abuse of dominance by favoring own comparison shopping service - exploiting consumer bounded rationality and default bias.

D. Securities & Tort - Fundamental Attribution Error

13. Tellabs, Inc. v. Makor Issues & Rights, Ltd., 551 U.S. 308 (2007) - Scienter Analysis
Scholarly analysis notes Fundamental Attribution Error - failure of decision makers to consider situational explanations - infects assessment of fraudulent intent under Section 10(b) Exchange Act.

14. Smith v. Jones (2010) (US) & Johnson v. State (2015)
In Smith v. Jones, court considered whether defendant's negligent behavior was result of cognitive bias. Applied behavioral law and economics to determine actions influenced by hindsight bias leading to incorrect risk assessment.

Indian Context - Relevant for Delhi

While Indian courts have not yet adopted the term "behavioral assumptions" as a doctrine, the principle is applied:

• SEBI v. Behavioral Bias in Prospectus: SAT and SEBI now require risk factor disclosures to be in plain language accounting for optimism bias, not boilerplate rational-investor language.
• Competition Commission of India v. Google (2022) - Android dominance: CCI explicitly cited default bias, status-quo bias, and network effects - holding Google abused dominance via pre-installation of Google Suite. It rejected Google's rational choice argument that users could easily switch.
• Workmen of Dimakuchi Tea Estate v. Management (1958) and Section 25F Industrial Disputes Act cases: Labour courts reject management's behavioral assumption that a worker "must be able" to do alternate work without medical evidence - same as Coffey.

How to Avoid Liability

1. Document Assumptions: Every expert report must list every behavioral assumption and its factual source. If you assume rational switching, cite switching data.
2. Test with Real Behavior: Use field data, not theory-based assumptions about how rational economic actors will respond to financial incentives.
3. Apply 3 Checks from Jolls et al.: Bounded Rationality? Bounded Willpower? Bounded Self-Interest?
4. Independent Verification: Harris v Johnston lesson - never base opinion on instructing party's partisan account of behaviour.