Sex Differences in Health: Evidence and Explanations 351suggest that the picture may be even more
complicated, such that the effect of HRT de-
pends on timing. There is the possibility that
HRT used by younger women or used closer
to menopause may be associated with reduced
risk (Rossouw et al., 2007).
Thus, a clear link between high levels of
estrogen and low levels of heart disease has
not been established. Another problem for
the theory is that oral contraceptives, which
often contain estrogen, increase risk factors
for heart disease. Oral contraceptives in-
crease blood pressure, cholesterol levels, and
blood glucose levels. In the past, using oral
contraceptives in combination with smok-
ing was a particularly lethal combination,
increasing the risk of a heart attack by a mag-
nitude of 30, but the synergy among more
recent classes of contraceptives has been re-
duced (Chasan-Taber & Stampfer, 2001).
Estrogen also plays a hazardous role
in the development of some cancers (breast
cancer, endometrial cancer) and may be
linked to osteoarthritis. Estrogens may play
a role in autoimmune diseases, but whether
the links are protective or harmful is not
clear. Thus hormones certainly play a role in
women’s and men’s health, but which hor-
mones are responsible for the effects and the
direction of the effects are not certain.Immune System
It has been suggested that the nature of
men’s and women’s immune systems differ
(Bouman et al., 2004), but the effects seem to
be paradoxical. Women’s immune systems
may respond to viruses better than men’s
(Whitacre et al., 1999), and women seem to
have a greater immune response to infection
than men (Rieker & Bird, 2005). However,
this immune response could explain why
women’s immune systems end up attacking
their own bodies resulting in a higher rateWhy does heart disease increase in
women after menopause? One theory is that
women are protected from heart disease be-
fore menopause because of their higher levels
of estrogen. With menopause, estrogen levels
drop. Although the decline in estrogen that
accompanies menopause does not influence
blood pressure, diabetes, or body mass in-
dex, it may lead to changes in cholesterol (i.e.,
decreasing the good cholesterol and increas-
ing the bad cholesterol), and it may alter the
blood clotting process (Fetters et al., 1996).
In the 1980s and 1990s, researchers were
so confident of the link between estrogen and
heart disease that many women were put on
hormone replacement therapy (HRT) after
menopause to reduce their risk of heart dis-
ease. However, most of studies linking HRT to
lower rates of heart disease were correlational,
meaning it was unclear whether HRT caused
a reduction in heart disease or whether there
was a third confounding variable, like SES,
that influenced rates of heart disease. That is,
women of a higher SES could have been more
likely to use HRT, and women with a higher
SES have better health. Finally, a randomized
trial of over 16,000 postmenopausal women
was conducted to determine the effect of HRT
on the prevention of heart disease (Writing
Group for the Women’s Health Initiative In-
vestigators, 2002). The trial was stopped early
in 2002 because the effects of HRT were so
dramatic. Unfortunately, the effects were not
as predicted. Women on HRT had a signifi-
cant increased risk of breast cancer and an in-
creased risk of heart attack. Subsequent trials
have linked HRT with an increased risk of
heart disease and stroke (Lowe, 2004). This
is a significant example of how important it
is to conduct experimental research to test
theories developed from correlational data.
We also have learned that higher SES women
were, in fact, more likely to use HRT (Lawlor,
Smith, & Ebrahim, 2004). More recent studiesM10_HELG0185_04_SE_C10.indd 351 6/21/11 8:54 AM