Inbreeding depression refers to the reduction in fitness traits due to inbreeding. This study reviews inbreeding depression (δ) in wild species, comparing it with captive-bred populations. Data from 169 estimates of δ for 137 traits across seven bird species, nine mammals, four poikilotherms, and 15 plants were analyzed. δ was corrected by dividing by F (inbreeding coefficient) to account for variation in F. The mean δ for homeotherms was 0.509 ± 0.081, for poikilotherms 0.201 ± 0.039, and for plants 0.331 ± 0.038. When corrected for F=0.25, wild estimates of inbreeding depression were significantly higher than captive ones, with wild estimates at 2.155 compared to 0.314 for captive species. Of the 169 estimates, 90 were significantly different from zero, indicating that inbred wild species often exhibit moderate to high inbreeding depression in fitness traits. Inbreeding depression is a significant factor in natural populations, with higher levels observed in the wild compared to captivity. This suggests that inbreeding depression is more severe in natural conditions, which has important implications for conservation and evolutionary biology. The study highlights the need for further research to understand the environmental factors influencing inbreeding depression and the potential for purging deleterious alleles in wild populations. The results indicate that inbreeding depression in wild populations is substantial and may contribute to population extinction risks.Inbreeding depression refers to the reduction in fitness traits due to inbreeding. This study reviews inbreeding depression (δ) in wild species, comparing it with captive-bred populations. Data from 169 estimates of δ for 137 traits across seven bird species, nine mammals, four poikilotherms, and 15 plants were analyzed. δ was corrected by dividing by F (inbreeding coefficient) to account for variation in F. The mean δ for homeotherms was 0.509 ± 0.081, for poikilotherms 0.201 ± 0.039, and for plants 0.331 ± 0.038. When corrected for F=0.25, wild estimates of inbreeding depression were significantly higher than captive ones, with wild estimates at 2.155 compared to 0.314 for captive species. Of the 169 estimates, 90 were significantly different from zero, indicating that inbred wild species often exhibit moderate to high inbreeding depression in fitness traits. Inbreeding depression is a significant factor in natural populations, with higher levels observed in the wild compared to captivity. This suggests that inbreeding depression is more severe in natural conditions, which has important implications for conservation and evolutionary biology. The study highlights the need for further research to understand the environmental factors influencing inbreeding depression and the potential for purging deleterious alleles in wild populations. The results indicate that inbreeding depression in wild populations is substantial and may contribute to population extinction risks.