|Fear, and correlative immobility, has been shown to be associated with reduced levels of endorphins in mice, who in such a condition or state tend to freeze and remain close to the walls (above), unable to explore their surroundings. |
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. 
In 1930, Subrahmanyan Chandrasekhar noted the following about Paul Dirac: 
“[Dirac is a] lean, meek shy young fellow [FRS] who goes slyly along the streets. He walks quite close to walls, and is not at all healthy. A contrast to Mr Fowler […] Dirac is pale, thing, and looks terribly overworked.”(add discussion)
The following are related quotes:
“We fear what we don’t understand.”— Aesop (c.550BC); Batman Begins (2005)“My object is to dispel the fear of the gods, which arises simply from the fact that there are so many things which men do not yet understand, and therefore imagine to be effected by divine power.”— Lucretius (55BC), On the Nature of Things“Courage is resistance to fear, mastery of fear—not absence of fear.”— Mark Twain (c.1880)
“The cave you fear to enter holds the treasure you seek.”— Joseph Campbell (c.1960), paraphrase (Ѻ) of his ideology, recorded by associate
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.
3. (a) Chandrasekhar, Subrahmanyan. (1930). “Letter to Father”, Oct 10.
(b) Farmelo, Graham. (2009). The Strangest Man: the Hidden Life of Paul Dirac, Mystic of the Atom (pg. 183). Basic Books.
● Pfaff, Donald W. (1999). Drive: Neurobiological and Molecular Mechanisms of Sexual Motivation (fear, pg. 163). MIT Press.
● Fear – Wikipedia.