which, by nessesity, means the parents aren't American either.
As the speaker didn't witness the birth of their own parents, the speaker simply does not know if they are Americans. It is not a joke about immigrants. As you correct state, lt is a joke about an unwillingness to believe what one did not personally witness.
During the 12 months leading up to March, more than 6,000 US citizens have applied to either become British subjects or to live and work in the country indefinitely
Cluster B disorders are marked by inappropriate, volatile emotionality and often unpredictable behavior. The disorders in Cluster B are antisocial personality disorder, borderline personality disorder, histrionic personality disorder, and narcissistic personality disorder.
Theoretically, turning in near the deadline gives time for one more editing session. The assignment could have been done days prior. Theoretically.