Insider Trading in SP Markets: What Academic Research Shows

Can You Read the Insider Signal in SP?

Horse racing has always lived with the suspicion that some participants know more than the market. Trainers know how their horse has worked at home. Jockeys know whether the instruction is to win or to educate. Stable staff know about that minor setback three days ago that has not reached the press. The question is not whether insiders exist — they do, by definition — but whether their activity is detectable in the SP, and how much it distorts the price you receive.

Academic research has attempted to quantify this using statistical models designed to measure information asymmetry in betting markets. A doctoral thesis from the University of Leeds by Chi Zhang, analysing data from Yorkshire racecourses across 2013–2014, applied the Shin measure — a widely used indicator of insider activity — and found that the level of informed trading in SP markets was only slightly lower than in comparable betting markets. What insiders know — and what the price reveals — is the central question of market integrity research.

The Shin Model: Measuring Information in Prices

The Shin model, developed by economist Hyun Song Shin in 1993, provides a framework for estimating the proportion of informed bettors in a market. The intuition is that in a market with no insiders, the overround is the bookmaker’s only source of edge. But in a market with informed participants — people who know, with greater certainty than the public, which horse will win — the bookmaker must widen the overround to protect against those informed bets. The degree to which the overround is inflated beyond what is needed to cover random variation indicates the level of insider activity.

In technical terms, the Shin measure (z) estimates the fraction of the betting pool that comes from informed sources. A z of zero would imply no insider activity at all — a perfectly uninformed market. A z of, say, 0.03 would suggest that approximately 3% of the money in the market comes from bettors with private information. Higher values indicate more insider activity; lower values indicate a more transparent market.

Zhang’s application of the Shin model to Yorkshire racing data produced z values that were slightly lower than those found in earlier studies of UK racing. This suggests a modest reduction in insider activity over time — consistent with the broader trend toward market efficiency driven by exchanges, data availability, and regulatory oversight. But the key word is “slightly.” The research did not find that insider trading had disappeared or become negligible. It found that it had declined marginally.

The Shin model has limitations. It cannot identify who the insiders are or what they know. It measures the aggregate effect of informed activity on prices, not the individual transactions. A low Shin measure could mean that insiders are few, or that they are many but their information is not significantly better than the public’s. The model detects the shadow, not the person casting it.

Key Findings From UK Studies

Zhang’s work sits within a broader body of UK-focused research on betting market efficiency and information flow.

Smith and Vaughan Williams, in a study published in the International Journal of Forecasting, examined ten seasons of UK Flat racing and found that the favourite-longshot bias — a related but distinct phenomenon — had diminished after 2000. While the bias is not the same as insider trading, both are indicators of market inefficiency. A shrinking bias implies that the market is getting better at pricing horses accurately, which in turn suggests that either insiders are less active, or their information is being incorporated into prices more quickly.

Earlier UK studies, conducted before the exchange era, found higher Shin measures — implying more insider activity in the 1990s than in the 2000s and 2010s. The common interpretation is that the arrival of Betfair in 2000 accelerated price discovery. On the exchange, large informed bets move the price immediately and visibly, alerting the rest of the market. In the traditional bookmaker ring, informed bets could be placed more discreetly, across multiple bookmakers, without the same immediate impact on the displayed price. The exchange made insider activity harder to hide.

Research from other jurisdictions broadly supports the UK findings. Studies of Australian, American, and Hong Kong racing markets all detect insider activity through the Shin measure, with the level varying by market structure, regulation, and the availability of exchange alternatives. The UK market, with its combination of on-course bookmakers, off-course firms, and a liquid exchange, sits in the middle of the global spectrum — not the most efficient market, but far from the least.

One consistent finding across studies is that insider activity is higher in certain race types. Maiden races and novice events — where horses have limited public form and the trainer’s private knowledge is most valuable — tend to show higher Shin measures than handicaps, where the form book provides abundant public information. This makes intuitive sense: the less the public knows, the more an insider’s edge is worth.

What This Means for SP Market Integrity

The academic evidence paints a nuanced picture. Insiders exist and participate in SP markets. Their activity is detectable but not overwhelming. The market is not perfectly efficient, but it is getting more efficient over time.

For SP integrity, this is broadly reassuring. The starting price is not being set in a market dominated by informed operators manipulating prices for private gain. The Shin measures from UK studies suggest that the vast majority of money — well over 95% — comes from uninformed or semi-informed participants. The SP reflects the aggregate opinion of a large, diverse market, not the private knowledge of a small cabal.

That said, “slightly lower” insider trading is not the same as no insider trading. At the margins, informed money does influence prices. A horse that drifts from 3/1 to 5/1 because connections know it had a bad scope the previous week is receiving a price that partly reflects private information — information the public punter does not have. The SP bettor in that scenario receives a longer price (5/1 rather than 3/1), which looks generous but in reality compensates for a genuinely reduced chance of winning.

The SPRC’s governance of the SP process provides a structural safeguard. By using a median of multiple bookmaker prices, the methodology filters out the effect of any single informed bet. An insider’s wager might move one bookmaker’s price, but the median of twelve or twenty-four prices is far more resilient to individual manipulation. The ISP’s expanded sample — incorporating off-course data — further dilutes the impact of targeted insider activity on any single board.

Practical Implications for Informed Bettors

If you are a serious punter — not an insider, but someone who studies form and monitors markets — the academic evidence offers several practical takeaways.

First, be cautious about reading too much into price movements. Not every drift is an insider signal, and not every steamer carries informed money. The Shin research suggests that the majority of price movement is driven by public information, tipster activity, and volume dynamics rather than private knowledge. Treating every move as an insider signal leads to chasing shadows.

Second, the market is most inefficient where public information is thinnest. Maiden races, first-time-out runners, and horses switching codes or surfaces are the categories where the Shin measure is highest — and where a punter with superior form analysis has the best chance of finding value before the market corrects. These are also the races where the SP is most likely to reflect an incomplete picture, because the market lacks the depth of form data that drives efficiency in handicaps.

Third, the exchange provides a partial antidote. BSP, derived from a peer-to-peer market with near-zero overround, is structurally less susceptible to the margin inflation that the Shin model attributes to insider activity. If you believe a race is likely to feature significant insider participation — a well-connected stable’s debutant, for example — taking BSP rather than bookmaker SP removes the bookmaker’s insurance against informed money from the equation. What insiders know is embedded in the price either way. What you pay for that price depends on which market you choose.