Inbreeding depression, where offspring from closely related parents have lower fitness, is usually rare in large, diverse populations. However, a study on marine stickleback fish found that offspring survival decreased when parents shared more genetic similarities. This suggests that inbreeding can lead to significant fitness loss even in outbred populations. Surprisingly, having some level of inbreeding in parents may actually improve offspring survival in certain cases. This study highlights the importance of considering genetic relatedness in understanding inbreeding effects in nature.