Only people known as "sensitives" could recognize a stranger.

Sensitives were rare; only one in every 100,000 people (0.001% of the total population) expressed sensitivity. This trait was linked to no known gene; only outsiders seemed to express it more readily. The age of onset varied — a select few displayed sensitivity from birth, while most developed this attribute during adolescence or later.

Sensitivity expressed itself differently among different people. Some sensitives felt the strangers' influence as flashes in their mind's eye, and the weight of dread. Others perceived strangers as ghosts, or holographic apparitions.

Some sensitives were very weak, perceiving only a vague few strains. Others were strong enough to see the menagerie around them with perfect clarity.

Sometimes, when there were no non-sensitives around, sensitives and strangers met one another face to face. It was in these lonely places that the sensitives were stalked, scrutinized, intoxicated, led astray, dismantled, burnt, drowned, and otherwise murdered.