Jonathan asked about the connection between intrapartum interventions and
perineal tears/lacerations.
Yes, Virginia, there is a connection:
1 - doctors are taught to believe an episiotomy is necessary.  Most of them
have never seen, much less been trained in, the art of assisting the
emergence of the baby's head in a slow, gentle, low tech-high touch way
2 - docs are taught to believe that  stretching (as opposed to cutting)  the
perineum is dangerous and leads to weakened muscles or a vaginal opening that
is too loose and not as pleasing to the male partner
3 - docs are in a hurry and don't want to waste their time; doing an
episiotomy can save a lot of time (cutting to the chase, as it were)
4 - drugged moms have reduced or no control over their ability to push (or
stop pushing  when indicated) in order to help with the controlled delivery
of the baby's head (assuming there is someone on the other end who knows what
they are doing) .
5 - drugged moms are much more likely to need forcep/vacuum extractor
assisted deliveries --- no time for careful and controlled now!
6 - drugged moms = drugged babies which creates higher incidence of stressed
babies that need to be delivered ASAP (this is where a lot of those 3rd and
4th degree lacerations occur)
6 - docs will tell you an episotomy is far less traumatic than an
uncontrolled tear (ask the question, "Why would the tear be UNCONTROLLED?")

I have been at both ends (literally) of beautiful, controlled births.  My
first baby was 8 1/2 lbs. (with a nuchal (hand) presentation - surprise!).  I
am 5' 2" and have a small frame.  Thanks to a wonderful, considerate, loving
midwife, my baby was born over an intact perineum after maybe 20 minutes of
perineal massage, manual stretching and keeping the baby's head well-flexed.
 The warm olive oil helped too.  I still have a soft spot in my heart for
Bertoli's. Meredith was born *between*  contractions.  After her head
crowned, the midwife had me bear down carefully and Meredith's head slipped
out.

I have been with friends during their hospital births where the moms had
asked the doc not to do an episotomy (response: "I won't do an episiotomy
unless it's necessary" and then they stand there watching the bulging
perineum with their hands practically in their pockets --- oops, looks like
you're going to need that episiotomy after all)

References::
"A Guide to Midwifery: Heart and Hands" by Elizabeth Davis.  John Muir
Publications.  pp. 102-3, 106-111.  Great photos on pp. 57 and 108.
Other good books are "Immaculate Deception" by Suzanne Arms and "Spiritual
Midwifery" by Ina May Gaskin.

These are the references I know about off the top of my head.  I am sure
there are Lactnetters out there with more recent sources and experiences,
especially the hospital based RNs.  Anyone care to join in?