The neural serotonin (5-HT)/serotonin transporter (5-HTT) system is involved in the regulation of physiological processes and emotional states. In humans, the short (S) allele in the 5-HTT gene-linked polymorphic region, which decreases 5-HTT expression, has been shown to be associated with behavioral changes including an increased level of anxiety. Also in birds a polymorphism in the 5-HTT gene is described, a deletion (D) has been found to have functional consequences on growth and locomotion. Furthermore, the D-allele leads to an increased 5-HTT expression compared to the wild type (W), a feature which is linked to lower levels of fear in mammalian species. Thus, we aimed here to test whether the polymorphism in the chicken 5-HTT gene also leads to respective alternations of fear-related behaviors. We tested 268 hens of three genotypes (W/W, W/D, D/D) in two behavioral paradigms (open field, light–dark test) to assess fear-related behavior. Both tests revealed that hens possessing the D-allele showed lower levels of fear than those having the W-allele. These similar outcomes in fear-related behaviors in an avian and a mammalian species are associated with an increased 5-HTT expression. In the human 5-HTT gene, the long (L) allele is linked to such increased expression, whereas in chickens it is the D-allele. Thus, increased 5-HTT expression causing decreased fear may be a general mechanism in vertebrates.