In science, fear is a state of reduced activity or immobility characterized by the presence or imminence of danger. [1] Neuro-biologically, fear has been shown to be associated with a hyperactivity in the amygdala, one of the four parts of the limbic system.

In 2001, American neurobiologist Donald Pfaff showed that fear is correlative to a reduced level of endorphins. Conversely, a high endorphin level is associated with fearlessness. Pfaff and associates showed that that genetically modified mice made to be incapable of producing endogenous opiates (lacking endorphins), behaved such that the smallest fright made the animals freeze, and while in large rooms they tended to say close to the walls, showing typical signs of fear. [2]

References
1. Fear – Dictionary.com.
2. (a) Ragnauth, A., Schuller, A., Morgan, M., Chan, J., Ogawa, S., Pintar, J., Bodnar, R.J. and Pfaff, D.W. (2001). “Female preproenkephalin-knockout mice display altered emotional responses.” Proc. Natl. Acad. Sci. USA, 98: 1958-63.
(b) Frobose, Gabriele, and Frobose, Rolf. (2006). Lust and Love: Is it More than Chemistry? (fear, pg. 107). RSC Publishing.

External links
● Fear – Wikipedia.