Backward, retrograde, retrogressive, regressive all involve the idea of not moving or going ahead, or forward, or in advance.
Only when applied to motion or a movement does backward imply the reverse of forward motion. Its commonest implication is failure to move ahead; in this sense it is chiefly applied to human beings who do not or cannot progress or develop with others of their age, kind, or class or to persons or things that hold back or are held back from doing what is normal or to be expected; thus, a child who is unable to keep up with others of his age in school because of some degree of mental deficiency is described as backward; a person who holds back from expressing his appreciation, or in urging his candidacy for a position, because of shyness or self-distrust is also describable as backward; when cold weather and frosts delay the development of vegetation beyond the normal or usual time, the season may be called backward.
Retrograde is not only applicable to backward motion and backward movement but also to any moving or seemingly moving thing that proceeds in a direction which is contrary to the direction usually followed by things of its kind.
It is also applicable to a process (as of natural development) in which the events occur in an order contrary to the usual or progressive; thus, an animal that passes from a more complex to a simpler and often degenerate state during development is said to manifest retrograde development.
Retrograde when applied to races, cultures, institutions, or movements differs from backward in implying decline or degeneration; thus, a backward society is one that does not progress, while a retrograde society is one that is relapsing into barbarism or sinking into an inferior state.
Retrogressive implies opposition to progressive. Like retrograde, and unlike backward, it implies movement in the direction that is the reverse of forward; unlike retrograde, however, it is seldom applied to physical movement; thus, one speaks of a retrograde (but not a retrogressive) movement or rotation, but one might speak of retrogressive (or retrograde) cruelties or behavior when stressing decline from some higher or more progressive level.
Retrogressive is sometimes preferred as a milder term when the reverse of improvement or betterment rather than positive decline from an improved or better state is implied.
Regressive carries a stronger implication of going backward by steps or degrees and often, also, a weaker implication of failure to progress or move ahead than any of the others.
Consequently it is often the preferred term when a colorless or uncolored statement of fact is intended; thus, when one infers a cause from an effect or a principle from a number of facts he follows a regressive process of reasoning; the process of growing old may be described as a retrograde development when the emphasis is on its backward direction, as a retrogressive development when the stress is on the absence of progress, and a regressive development when the intent is to indicate that it is marked by an inversion of order in its stages; a regressive loss of memory implies that the most recent memories disappear first and the earliest linger longest.