# Why do independent priors for two random variables not result in an independent joint posterior distribution?

Suppose $X$ and $Y$ are independent random variables with prior distributions $p_X(x)$ and $p_Y(y)$. Suppose now that we have data $D$ that are characterized by $X,Y$ and that after observing the data, we have a posterior distribution:

$$p(X,Y|D)$$

Why is it that two independent priors do not result in a posterior distribution that is also independent, or that $p(X,Y|D) \neq p(X|D)p(Y|D)$?

Is there a further implication here in terms of real-life applications? Thanks.

When you have an independent prior on $X$ and $Y$, then the posterior might not factor into $X$ and $Y$ pieces just because the likelihood doesn't factor into $X$ and $Y$ pieces.
It's easy to see that $$p(x,y|D) \propto p(D|X,Y)p_X(x)p_Y(y).$$ So in your situation the posterior factors if and only if the likelihood factors.
• Can the likelihood ever even factor? I say this because we have that $p(D|X,Y) = \frac{p(D,X,Y)}{p(X,Y)}$ and so I don't see how we can ever get $p(D|X)p(D|Y)$ as this would require two "copies" of $D$? Jun 26 '17 at 5:27
• @user321627 I can't think of any univariate examples, but $D$ could be a 2-d vector of conditionally independent random variables, each dependent on only one of the parameters. $p(d|x,y) = p(d_1|x,y)p(d_2|x,y) = p(d_1|x)p(d_2|y)$ Jun 29 '17 at 13:20
• @user321627 your reasoning is flawed -- $p(D|X,Y) =p(D|X)p(D|Y)$ does have solutions -- $\frac{P(D,X,Y)}{P(X,Y)} = \frac{P(D,X)}{P(X)}\frac{P(D,Y)}{P(Y)} \implies P(D,X,Y) = P(D,X)P(D,Y)$. Any joint distribution that follows this last factorization works. You do not have to worry about there being two Ds because there is no linearity in the probability distribution function. i.e $P(X)$ is not$\propto X$ Apr 29 '18 at 18:51