What fraction of the population has secure attachment style? Attachment theory suggests that we divide attachment styles into secure attachment style vs insecure attachment styles.

What fraction of the population would be classified as having a secure attachment style, and what fraction as having an insecure attachment style?  Is there are any data?

If there is any data on the breakdown between different types of insecure attachment style, that would also be interesting.  I am most interested in the US or Western cultures, if it is culturely-dependent, but I would be interested in any references.

I found a reference that describes Mary Ainsworth's initial study of infants from middle-class Baltimore families as having the following proportions: 66% secure attachment (type B), 20% insecure-avoidant (type A), 12% insecure-ambivalent (type C).  I found a reference to a 1992 study from Fonagy et al. that seemed to suggest a split of about 57% secure attachment, and 42% insecure attachment.  Another website reports the proportions as 70%, 15%, 15%.  That's a fair bit of variation among numbers.
Welcome to Psychology.SE. The answer to your question is not that straight forward. You could have a strong attachment style amongst work colleagues but have an insecure attachment style with superiors or parents.

For different types of attachment (insecure/secure) I would suggest reading the works of John Bowlby who pioneered the theory of attachment.

For further reading there is also a book called Understanding Disorganized Attachment: Theory and Practice for Working with Children and Adults by David and Yvonne Shemmings