Assumption of innocence
In a trial, jurors are expected to assume the defendant is not-guilty. But doesn’t the fact that they have been arrested already mean that they are probably guilty?
Is the knowledge that someone has been arrested itself evidence?
This logic is based on Bayes Theorem which is used to calculate conditional probabilities (What is the probability of X given Y). For example, in the case of a person on trial for drunk driving, what is the probability that they were drunk given that they were arrested?
Suppose hypothetically:
Bayes’ theorem says:
Probability of being drunk, given that defendant was arrested is P(D)/A) :
P(D/A) = {P(A/D) x P(D)} / (P(A))
Where:
G= actually guilty of drunk driving
A= arrested for drunk driving
Example, lets say:
1% of drivers are actually drunk: P(D)=0.01
Police correctly arrest 90% of drunk drivers: P(A/D)=0.90
Police falsely arrest 0.1% of sober drivers: P(A/not D)=0.001
Then the probability of being arrested P(A)
P(A) = 0.90(0.01)+0.001(0.99)
= 0.009 + 0.00099
= 0.00999
Now apply Bayes:
P(D/A) = (0.90×0.01)/0.00999 ≈ 0.901
So the probability the person is actually guilty given the arrest would be about: 90.1%
So just by the fact they were arrested means they are probably guilty.?
Is the knowledge that someone has been arrested itself evidence?