Two phenotypically distinct lines were created, selecting for high or low group productivity and survival. Each line developed unique behavioural and physiological characteristics, when housed in a multiple-bird cage system. This study examined how the variation in the adaptation of the two lines has affected the control of the plasma dopamine concentrations and how the adrenal gland responds to social stress. At 17 weeks-of-age, laying hens from the HGPS and LGPS lines were housed in single cages or cages that housed 10 hens. The effect that the genetics and housing had on the dopamine concentration and adrenal function was measured on the 24th week-of-age. When 10 hens were housed per cage, the HGPS hens had lower dopamine concentrations and heavier adrenal glands. There was not a significant difference in the cortisol concentrations of the two genetic lines. However, in the single cages system, both lines had higher dopamine and lower cortisol concentrations than those hens that were housed in groups. The increase in the dopamine concentrations was significantly larger in the LGPS than the HGPS hens that were housed individually. As well, the adrenal glands were lighter in individually housed LGPS hens. The experiment found that the hormonal response to stress is different in the two genetic lines. It was also proven that environment affects the stress reaction. Thus, hormones and the environment are likely what causes the behavioural and physiological differences exhibited by the two genetic lines.