The most common explanation for why people with pectus excavatum have an indent in their chest is an overgrowth of costal (chest) cartilage. This is simply not true, and although several studies have disproved this, the concept is still everywhere and even clinicians still spread this false information.
Not a single study has provided evidence for this hypothesis. Here are three studies that disprove this hypothesis:
Study 1: Eisinger et al., 2019
In this study, 16 people with pectus excavatum were compared to 16 age- and gender-matched controls (people without pectus excavatum). The authors found that in ribs 4 through 8, people with pectus excavatum had shorter cartilage (NOT longer) relative to bone length.
Study 2: Nakaoka et al., 2010
In this study, 20 people with prepubertal pectus excavatum were compared to 24 similarly aged healthy controls without pectus excavatum. They looked at only the 5th and 6th costal cartilages and ribs, but they found a similar result as above. That is, people with pectus excavatum do not have larger costal cartilage:bone ratios.
Study 3: Nakaoka et al., 2009
This study included 24 adolescent and adult patients with asymmetric pectus excavatum and again looked at the 5th and 6th ribs. In fact, they found that the side of the chest with a more significant depression had shorter costal cartilage lengths.