I received the following article from HealthCentral.com.

David Stankov, San Dimas, CA.

 - - -

Speakers Of Mandarin, Vietnamese Often Have Perfect Pitch
November 12, 1999

If you think perfect pitch is a rare trait, possessed only by musical
geniuses such as Mozart, you'll be surprised to learn the results of a
new study.

Researchers at the University of California at San Diego say perfect
pitch is fairly common in people who speak in tonal languages such as
Vietnamese or Mandarin.  When Vietnamese and Mandarin speakers were
asked to read a list of words in their native language, they displayed
remarkably precise, perfect or absolute pitch, the researchers say in a
report to the Acoustical Society of America.

I've been to Asia and remember how the same word in Mandarin, such as
"Ma" can have four meanings depending on the tonal differences.

In this study, researchers used a computer to analyze the variation
in pitch of participant voices as measured in semitones - or the
difference between notes on a chromatic scale. Vietnamese speakers had
an average difference of less than 1.1 semitone and Mandarin speakers a
difference of less than 0.5 semitone.

This study contradicts previous research that showed fewer than one in
10,000 people having perfect pitch.  It also indicates that many people
have the potential for acquiring perfect pitch if they're trained in
tonal languages during childhood.