The costoclavicular (rhomboid) ligament connects the first rib to the clavicle, stabilizing the pectoral girdle. It produces skeletal traits that may be tubercles, roughened impressions, shallow groove-like fossae, deep fossae, or leave no trace. A pit or depression at this site is often called a “rhomboid fossa.” While these markings may appear pathological, they are normal variants of the clavicle. Using a large contemporary sample (N = 344; 113 females, 231 males), we evaluated the presence of a rhomboid fossa as a sex and age indicator for unidentified skeletal remains.

Logistic regression found significant relationships between the presence of a rhomboid fossa and sex and between presence of a rhomboid fossa and age. Fossae were more common in males (36% left, 31% right) than in females (3% left, 8% right). Posterior probabilities suggest that a fossa on the right clavicle is indicative of a male with 81.7% probability; a fossa on the left is indicative of a male with 92.2% probability. Younger individuals more commonly exhibited rhomboid fossae than older individuals, and the largest fossae were most common in males 20–30 years of age. However, the age effect was not conclusive and must be corroborated by other methods. A test of the sex estimation method on an independent sample (26 males, 23 females) found nine males and only one female with fossae present on the left clavicle.

When the costoclavicular attachment exhibits an impression, a tubercle, or leaves no trace, this method cannot be used for sex estimation. When a clavicle exhibits a rhomboid fossa, it is likely from a male. The greater difference in fossa expression between the sexes on the left clavicle makes use of the left bone preferable. This technique can corroborate other sex estimates or provide an estimate for unknown individuals in the absence of other skeletal indicators.