The Fourth Turning by William Strauss and Neil Howe (pg. 79)

If a generation’s shadow is two phases of life older (or younger), then a generation’s matching archetype is four phases of life older (or younger). “It is one of nature’s ways,” Igor Stravinsky once observed, “that we often feel closer to distant generations than to the generation immediately preceding us.” The affinity between grandparent and grandchild is universal folk wisdom.