What do DBT and ACT add to cognitive behavioral therapy, and is there really a "third wave"?

Dialectical behavior therapy (DBT) and acceptance and commitment therapy (ACT) are the best-known members of a group that some call the "third wave" of cognitive behavioural therapy.

DBT began with a failure. Standard CBT protocols of the late 1970s did not work for chronically suicidal clients, and Marsha Linehan developed a new approach, which she defines as "a synthesis or integration of opposites". It combines CBT techniques with distress tolerance, acceptance and mindfulness drawn from contemplative meditation. Many treatments at the time left patients feeling "criticized, misunderstood, and invalidated" because they focused on changing cognitions and behaviours, so the DBT therapist aims to accept and validate the client's feelings while showing better alternatives. The first randomized trial showed fewer suicidal gestures, hospitalizations and dropouts than usual treatment.

ACT, developed by Steven Hayes in the 1980s, starts from the claim that human suffering is created by human language and by avoiding unpleasant feelings. Rather than teaching people to control their thoughts and feelings, it teaches them to "just notice", accept and embrace them, and to act on their values. More than 1,000 randomized trials of ACT had been run by 2021, though critics such as psychologist James Coyne (2012) and William O'Donohue and colleagues (2016) have questioned how rigorous some of them were.

Is it really a new wave? Hayes grouped them that way, with behaviour therapy as the first wave (1920s) and cognitive therapy the second (1970s). But David Dozois and Aaron Beck argued there is no "new wave", only extensions of cognitive therapy, and critics say third-wave therapies "display an annoying tendency to gather effective methods from other traditions and label them as their own".