The Hypothesis — The Payday Effect

The original theory

"Did you notice that the stock market takes massive jumps on the days leading up to the influx of institutional money from 401(k)s, etc. for people that get paid on the 15th/end of month and every two-week cycles? Seems like collusion to drive up prices for regular people."

This is the claim we set out to test. Before we can run any statistics, we need to unpack the assumptions baked into it and turn them into concrete, measurable predictions.

Embedded premises

The theory contains four implicit claims, each of which must be true for the overall argument to hold:

Predictable calendar: Semi-monthly (15th and end-of-month) and biweekly paydays create a known, repeating schedule of money flowing into the stock market via 401(k) contributions.
Measurable price impact: The volume of money flowing in on these dates is large enough to move prices in a statistically detectable way.
Anticipatory run-up: Prices rise before the money arrives, not just when it lands — implying that someone is buying ahead of the known flow.
Intentional front-running: The anticipatory price movement is not random — it reflects deliberate action by institutional traders who know the calendar and trade ahead of it to profit at the expense of 401(k) participants.

Testable predictions

We translated the theory into five specific, falsifiable predictions. Each one maps directly to a statistical test:

ID	Prediction	What we measure	"Supports theory" if...
P1	Prices rise in the 5 trading days before paydays (T-5 to T-1)	Mean daily return in the run-up window vs. all other days	Run-up returns are significantly positive after controlling for known calendar effects (p < 0.05)
P2	Prices rise on payday (T) and the 3 days after (T+1 to T+3)	Mean daily return on payday and post-payday days vs. all other days	Payday and post-payday returns are significantly positive
P3	Volatility is elevated around paydays	Parkinson volatility in payday windows vs. non-payday days	Intraday range is wider around paydays than on ordinary days
P4	Volume is elevated around paydays	Trading volume on payday-window days vs. non-payday days	Volume is significantly higher in the payday window
P5	The price moves happen overnight, not during the trading session	Overnight return (prior close to open) vs. intraday return (open to close)	Overnight returns account for most of the excess return in the payday window

We are going to check whether the stock market behaves differently around paydays compared to ordinary days. Specifically, we will look at whether prices go up more than usual in the days before and around paydays, whether the market is choppier or busier on those days, and whether any price gains happen overnight (when regular people are not trading) rather than during the day.

What would DISprove the theory

A hypothesis is only useful if we can specify what evidence would cause us to reject it. The theory would be disproven if:

No return difference: Average daily returns in the run-up window (T-5 to T-1) are statistically indistinguishable from returns on all other days, even after controlling for the turn-of-month effect and other known calendar anomalies.
No volume or volatility signature: Trading volume and intraday volatility around paydays are no different from any other period.
Pattern is constant across eras: If the pattern existed equally before and after 401(k) plans were created (pre-1980), it cannot be caused by 401(k) flows.
No schedule sensitivity: If weekly, biweekly, and semi-monthly paydays all produce the same signal (or none at all), there is no evidence that the concentration of flow matters.
Fails permutation test: If random date shuffles produce patterns as strong as the real payday calendar, the "effect" is likely noise. A Monte Carlo p-value above 0.05 would fail to reject the null hypothesis.

Possible alternate explanations

Even if we find a statistically significant pattern, it does not automatically mean the theory is correct. We must consider alternative explanations:

Turn-of-month effect

The turn-of-month effect is a well-documented anomaly (Ariel 1987, Lakonishok & Smidt 1988) where stock returns are disproportionately concentrated in the last trading day of each month and the first three days of the next month. Because end-of-month paydays overlap with this window, any payday signal could simply be the turn-of-month effect wearing a different label. Our regression controls explicitly for this.

No signal at all

The most parsimonious explanation is that there is no payday effect — that daily returns around paydays are indistinguishable from any other trading days once known calendar effects are removed. This is our null hypothesis, and the burden of proof is on the theory to disprove it.

Volatility compression

It is possible that what looks like a payday "bump" is actually reduced volatility around paydays creating an illusion of smoother upward drift, while the magnitude of returns is unchanged. Our GARCH analysis helps disentangle return effects from volatility effects.

Revision log: The hypothesis evolved during the investigation. The original theory assumed money hits the market on the paycheck date itself. After initial tests showed no signal at T=0, we investigated clearing lag — the 3-7 business day delay between a paycheck and when the 401(k) contribution actually settles into stock purchases. Shifting the calendar forward by 7-8 trading days revealed a pattern that was invisible with the naive calendar. This iterative refinement is documented throughout the site.