Introduction

At the 2015 yearling sales full-siblings to a number of talented racehorses were offered for sale. These included a full-sister to Group 1 winning filly Mecca’s Angel, sold for £825,000, a full brother to Group 1 winning colt Prince Gibraltar, sold for €100,000 and a full sister to dual Arc winner Treve, led out of the ring unsold at €1.2m.

The attraction of younger siblings is clear to both buyers, hoping to replicate the success of the older sibling, and to breeders, expecting juicy prices at the sales.

In the analysis presented below we compare full-sibling racecourse performance, and compare with half-siblings performance from the same dam.

Analysis

Using RaceForm (RF) data,  4,334 sibling pairs that had reached the racecourse were identified. Since we are most interested in sibling pairs when the eldest sibling has performed well on the racecourse, the pairs were filtered  to those where the eldest has performed to a Racing Post Rating (RPR) of at least 100. In Table 1 below the median RPR and percentage of the younger siblings to reach an RPR of 100 are shown. There are 984 qualifying pairs, and the median rating of the younger sibling is 84, 25lb worse than the median rating of the elder sibling group, with ca. one-quarter going on to achieve an RPR in excess of 100.

Sibling                      Qualifiers            Median RPR                    Pct RPR100

Elder                              984                             109                                            100.0%

Younger                        984                               84                                             25.4%

Table 1: Full-sibling performance compared

So how does this compare with half-sibling performance from the same set of dams? In Table 2 below the performance of half-sibling pairs where the eldest half-sibling ran to an RPR of 100 are shown. Dams were restricted to the 4,334 that produced the sibling pairs that formed the basis of the analysis in Table 1. There are 497 qualifying pairs, and the median rating of the younger sibling is 80, 28lb worse than the median rating of the elder sibling group, with ca. one-fifth going on to achieve an RPR in excess of 100.

Half-Sibling       Qualifiers                Median RPR                    Pct RPR100

Elder                              497                             108                                            100.0%

Younger                        497                               80                                             19.1%

Table 2: Half-sibling performance compared

Summary

Buying a full-sibling to a racehorse that has achieved an RPR of 100 has given you about a one-quarter chance of repeating the outcome, and has improved your chances relative to buying the half-sibling to an RPR100 horse by ca. 30%. By way of comparison, ca. 7.7% of lots sold at Tattersalls Book One in 2007 went on to achieve pattern class success.

Given the difference in performance full- to half- siblings is just 4lb, it is possible this difference could be explained by differences in the ability of the sires visited by the dams.

Further analysis looking into sire differences, as well as an investigation of prices paid for younger siblings, would be of interest.