all and that hypotheses about such functions can be tested just like any others (Mayr 1974). Complex traits can be shaped by natural selection only if they Subtypes of Anxiety

serve functions that increase fitness. Hypotheses about these functions are not matters for speculation but for clear fonnulation and rigorous testing. Testing of evolutionary hypotheses is now the focus of most research into animal behavior (Alcock 1989), especially by behavioral ecologists (Krebs and Davies 1991), who emphasize the functional significance of behavior, not just A question of major concern is how to split (or lump) the various kinds anxiety disorders, and how to justifY the taxonomy. Some researchers empha- size the similarities of all anxieties and postulate the unity of all anxiety disorders. Others stress the differences between different kinds of anxiety, positing several distinct disorders, each with its own etiology, phenomenology,