The lock-up period is a contractual agreement between a company's insiders — executives, employees, and pre-IPO investors — and its IPO underwriters that prohibits the sale of shares for a specified period after the initial public offering, typically 90 to 180 days. When the lock-up expires, insiders who collectively hold the majority of the company's outstanding shares become free to sell for the first time. The resulting supply shock — millions or billions of dollars in shares flooding a market that had been operating with artificially constrained supply — produces predictable, measurable, and investable price declines that the informed investor can anticipate.
The Supply Mechanics
At the moment of an IPO, the shares available for public trading — the float — represent a fraction of the company's total outstanding shares. A company with 500 million shares outstanding may offer 50 million in its IPO, creating a float of 10 percent. The remaining 450 million shares are held by founders, executives, employees, and venture capital and private equity investors, all of whom are contractually locked for 90 to 180 days.
During the lock-up period, trading occurs exclusively within the constrained float. If demand for the stock is strong, the limited supply amplifies price appreciation — the same buying pressure distributed across 50 million shares produces a larger price increase than it would across 500 million shares. This artificial scarcity contributes to the post-IPO price gains that characterize many recent offerings.
When the lock-up expires, the tradeable supply expands by a factor of five, ten, or more. Insiders who have held illiquid paper wealth for years — in some cases, for a decade or more of private company growth — face their first opportunity to convert that wealth into cash. The selling pressure is not speculative. It is mechanical. Employees with RSUs that vested at pre-IPO valuations face tax obligations triggered by the vesting event. Venture capital funds face contractual obligations to return capital to their limited partners. Founders face the rational desire to diversify concentrated positions that may represent 90 percent or more of their personal net worth.
The Empirical Record
Academic studies of post-lock-up price behavior, spanning thousands of IPOs across multiple decades, consistently find statistically significant negative abnormal returns in the period surrounding lock-up expiration. The average decline ranges from 1.5 to 3.5 percent in the two weeks surrounding the expiration date, with larger declines — 5 percent or more — for companies where insider holdings represent a larger multiple of the public float and where pre-expiration price appreciation has been strongest.
The decline is not a panic. It is a market equilibrium adjustment. Before the lock-up, the share price reflected supply and demand within the constrained float. After the lock-up, the share price reflects supply and demand within the full outstanding share count. The fundamental value of the company has not changed. The denominator against which that value is distributed has.
Critically, the market does not fully anticipate the lock-up expiration in advance. Despite the fact that lock-up dates are publicly disclosed in the IPO prospectus (Form S-1) and can be calculated by any investor with a calendar, the negative price impact persists — suggesting either that retail investors are unaware of the mechanism or that the magnitude of insider selling is uncertain enough to prevent perfect price adjustment.
How to Trade Around Lock-Up Expirations
The investor considering a post-IPO stock should determine three things before committing capital. First, when does the lock-up expire? The date is disclosed in the S-1 prospectus, typically in the section titled "Shares Eligible for Future Sale." The standard period is 180 days from the IPO date, but modified lock-ups with staggered release schedules — where different tranches of insider shares unlock at different intervals — are increasingly common.
Second, how large is the insider overhang relative to the float? If insiders hold 400 million shares and the float is 50 million, the potential supply expansion is eight-fold. If insiders hold 100 million shares and the float is 200 million, the expansion is more modest and the price impact will be correspondingly smaller.
Third, what are the insiders' incentives to sell? If the stock has appreciated significantly since the IPO, insiders have unrealized gains and tax-driven incentives to sell. If the stock has declined, insiders may choose to hold in expectation of recovery, reducing the actual selling pressure despite the legal right to sell.
The disciplined approach for a retail investor interested in a recently public company is to wait. Allow the lock-up to expire. Allow the insider selling to occur. Allow the price to find its post-expansion equilibrium. Then evaluate whether the company's fundamentals justify investment at the lower, more accurately priced level. The investor who buys before the lock-up expiration is paying a premium for artificial scarcity. The investor who buys after is paying for the company's actual worth in a fully liquid market.
Early Lock-Up Releases: The Exception That Proves the Rule
In some cases, underwriters grant early lock-up releases — permitting insiders to sell before the contractual expiration date. This typically occurs when the stock price has risen substantially and the underwriter (who also serves as the lead book-runner for any secondary offering) sees an opportunity to manage an orderly sale. Early releases are disclosed through SEC filings and press releases, but they often catch retail investors by surprise because they arrive ahead of the expected expiration date.
The investor should monitor SEC filings (Form 4, which discloses insider transactions, and Form 144, which signals an intent to sell restricted shares) in the weeks preceding the expected lock-up expiration. A cluster of Form 144 filings is an advance signal that insiders are preparing to sell — and that the supply shock may arrive earlier than the calendar suggests.