Bayes’ theorem tells us
prior experience should alter
future expectations.

Four hundred cycles
ought to count
as compelling evidence.

The data
are overwhelming.

Somehow,
every month,
my brain
cleans the slate.

It becomes convinced
the sadness
is evidence.

That this loneliness
means something
permanent.

That my friendships
are not
what I thought.

That I’m getting
old.

You would think
the confidence interval
would narrow.

Instead,
my state of mind
remains wildly
unimpressed
by the data.

Which makes me wonder
whether Bayes
gave us
too much credit.

We like to imagine
evidence
changes minds.

History suggests
otherwise.

We call
monthly repetition

a hormonal cycle.

Century-long repetition
becomes

world history.

Nations.

Elections.

Marriages.

Families.

We repeat ourselves
with astonishing confidence,

convinced
this time
the conclusion
is different.

Twenty-six days later,

the experiment
is repeated.

The results are nearly identical.

Scientists can explain
the chemistry.

Estrogen.

Progesterone.

A conversation
between molecules
that somehow becomes
a conversation
with myself.

The tears
are real.

The exhaustion
is real.

The loneliness
is real.

What changes
is not the feeling.

Only the story
my brain constructs
to explain it.

Now my daughter
is approaching
the age

when I will have
to teach her
about all of this.

Not simply how periods work.

But how
a belief
can arrive

wearing
the costume
of truth.

How do I tell her
that her mind

will occasionally
make extraordinary claims

with very little
new evidence?

That she should trust her feelings,
but perhaps
postpone trusting
their conclusions.

That biology
is sometimes

an exceptionally persuasive
storyteller.

I hope I remember to tell myself
the same thing.

Because next month,

despite
approximately
four hundred
previous experiments,

despite all the graphs,
all the chemistry,
all the evidence,

I will almost certainly
believe it again.

And perhaps that is the strangest part.

Not that hormones
can change
us.

But that evidence
remains
so strangely powerless
against certainty.