A lockup expiration (or "unlock") is the date on which the contractual restriction barring a newly public company's insiders — founders, employees, venture and private-equity backers, and pre-IPO investors — from selling their shares comes to an end. The lockup is a private agreement (not a securities-law requirement), most commonly running 180 days from the IPO, written into the underwriting agreement to prevent insiders from flooding a thin post-IPO float and to signal confidence to public buyers. The unlock is therefore a scheduled, known-in-advance supply shock: on a single calendar date, the number of shares legally available to sell can multiply — often by several times the existing public float. The core tension is that the date and the rough magnitude are public, yet markets still tend to react, which makes lockups a recurring test case for whether anticipated supply events are efficiently priced.

How it's structured

• Standard term. ~180 days is the modal duration; 90- and 365-day terms exist, and very short or staggered terms have become more common since ~2018. The term and exceptions are disclosed in the IPO prospectus (S-1) "Shares Eligible for Future Sale" section.
• Who is locked. Affiliates and most holders of restricted stock. Even after a contractual lockup lifts, U.S. affiliates (officers, directors, 10%+ holders) remain subject to SEC Rule 144 volume caps (broadly, the greater of 1% of shares outstanding or the average weekly trading volume over the prior four weeks, per rolling 90-day window) and Form 144 filings. Rule 701 governs pre-IPO employee equity. So "unlock" rarely means all shares can hit the tape at once.
• Underwriter discretion. The lead bookrunner can waive or release the lockup early, or extend it. Early waivers and "blackout pull-forward" releases (timed just before earnings blackouts) are at the underwriter's discretion.
• Modern variants (per practitioner guidance, e.g. StrikeRates): day-one liquidity for a small slice of vested employee shares; staggered tranches (e.g., portions released at intervals tied to quarterly earnings); and price-based triggers (shares unlock only if the stock holds a premium — commonly cited around 25–33% above the IPO/reference price — for a sustained number of trading days). Staggering deliberately spreads supply across weeks instead of one cliff.

How it's used in practice

Traders and analysts treat the unlock as a calendar-driven supply/catalyst window rather than a directional signal in isolation. The practical workflow:

1. Read the S-1 to get the exact date(s), the number of shares unlocking, and who holds them (VC/PE sponsors vs. dispersed employees). 2. Size the shock as a multiple of free float — an unlock of 4–5x the current float is materially different from one of 0.5x. The bigger the locked overhang relative to daily liquidity, the more meaningful. 3. Identify motivated sellers. VC and PE funds face their own redemption clocks and distribution pressure and historically sell more aggressively than executives. Employees with concentrated, appreciated positions are also natural sellers. 4. Watch for the waiver / extension headline, which is itself a catalyst (early release = bearish surprise; extension = supportive).

Pre-event behavior also matters: there is evidence of elevated short selling ahead of unlocks, and option-implied volatility often reflects the date.

Adoption, debate & evidence

This is one of the better-studied "anticipated event" effects in finance, and the honest picture is small, real, but largely front-run.

• Field & Hanka (2001, Journal of Finance), studying 1,948 lockups, document a statistically significant three-day abnormal return of about −1.5% and a permanent ~40% jump in average trading volume after the unlock. The effect is larger for venture-backed and high-tech firms.
• Bradley, Jordan, Roten & Yi (2001, Journal of Financial Research) confirm that the negative returns are concentrated in VC-backed firms, where abnormal returns and volume are markedly larger and VCs sell most aggressively; non-VC firms show weaker effects. Losses cluster in firms with the biggest post-IPO run-ups.
• The puzzle: a known date producing a negative abnormal return is, on its face, inconsistent with the efficient-market hypothesis. This is the central debate. Competing explanations include a downward-sloping demand curve / price-pressure from the supply increase (Ofek & Richardson and others), liquidity/bid–ask-spread shifts, and the limits of arbitrage given short-selling frictions. There is no full consensus on mechanism.
• Magnitudes vary across samples and eras; later studies report smaller, sometimes statistically marginal, effects, consistent with the market increasingly anticipating well-telegraphed unlocks and with staggered structures diluting the single-day cliff. Treat the Field-Hanka ~−1.5% average (and the somewhat larger losses documented for the VC-backed/high-tech subset) as a historical central tendency, not a reliable forward bet.

Folklore-vs-measured: the trading-desk lore that stocks "always dump into the lockup" overstates a modest average that is dominated by a minority of high-float, VC-heavy, post-run-up names — and that is often already reflected in the price by expiration day.

Strengths & limitations

When it's informative: large overhang relative to float; concentrated motivated holders (VC/PE); a stock that has run up sharply post-IPO; a single hard cliff rather than staggered tranches. These are the conditions under which the documented effect is strongest.

When it fails / misuse: The #1 misuse is treating the unlock as a guaranteed short. The effect is small, well-known, front-run, and frequently swamped by company news, broad-market moves, or simple absence of selling (insiders need not sell, and many don't). Idiosyncratic outcomes dominate any single name. Other traps: ignoring Rule 144 caps that throttle affiliate sales; missing an underwriter waiver/extension that changes the date; and assuming all locked shares are for sale when much of the overhang is held by long-term holders or executives subject to 10b5-1 plans.

Sources

• Field, L.C. & Hanka, G. (2001), "The Expiration of IPO Share Lockups," Journal of Finance 56(2): 471–500 — three-day ~−1.5% abnormal return, ~40% volume rise. (Wiley / RePEc: ideas.repec.org/a/bla/jfinan/v56y2001i2p471-500.html)
• Bradley, Jordan, Roten & Yi (2001), "Venture Capital and IPO Lockup Expiration: An Empirical Analysis," Journal of Financial Research — losses concentrated in VC-backed firms; VCs sell most aggressively. (Wiley; SSRN abstract=238714)
• Ofek, E. & Richardson, M., "The IPO Lock-Up Period: Implications for Market Efficiency and Downward Sloping Demand Curves" (downward-sloping-demand / price-pressure explanation). (ResearchGate 2392799)
• "Short selling around the expiration of IPO share lockups," Journal of Banking & Finance (ScienceDirect S0378426617302339) — pre-event short interest.
• StrikeRates, "IPO Lock-Up Periods: The 180-Day Selling Restriction" — mechanics, staggered/price-based/day-one structures, underwriter discretion. (strikerates.com/learn/ipo-lockup)
• SEC Rule 144 / Rule 701 — affiliate volume limits and Form 144 filing requirements.

Dispute flagged: The existence of a negative abnormal return around a known date is debated against market efficiency; mechanism (price pressure vs. information vs. liquidity) is unresolved, and effect sizes have likely shrunk over time as unlocks are anticipated and staggered.