Usual Answer: Because the child looks "more black" than white.
Follow up question: Why does the child look more black?
Follow up answer: Because whites have recessive genes."
Awhile ago on http://www.contexts.org/socimages, I saw a post that summed up my feelings about the "First Black President". She posted images of the president's grandfather and of Obama as an adult. Here are those two pictures:

And:

Clearly, Obama is the splitting image of his grandfather. So what about him makes him 'look black' exactly, out of curiosity? Is it complexion, features, or because "we know"? When we assign people to a race (and when it's ambiguous we ask 'what are you?' so we can attach stereotypes to 'understand them better') what are we really looking at? Or is it that a biracial person is said to look "more black than white" because OUR definition of whiteness is much less vague and always has been, than blackness? The "octaroons" during colonization and slavery show that white superiority has always meant that blackness was "too dirty" to rub across it. A person who was 1/8th (and prior to that 1/16th) black was "tainted" and therefore left out of the construct of white privilege as punishment.


How strange it is that people who range from Alicia Keys complexion to Wesley Snipes complexion are considered to "look black"? Especially since Alicia Keys is much lighter than Osama Bin Ladin, who is "racially" considered caucasian, as is Angelina Jolie who has features associated with black women. What about those who range from say Zoe Saldana to Cameron Diaz, how can you safely say you can "tell" someone "looks" latina? This has always been a huge WTF and a big SMH, imho.
We can use "culture" to define who is "black" and "white", but most biracials are raised by their white mothers so by this argument, why aren't they considered "white"? Then, there's the fact that other than stereotypes, underprivilege, and isolation there's nothing in America unifying "blacks" to each other to create a distinct culture. There's also the issue that states that biracials have "trouble" with their identity without considering the same ramifications exist for someone who is "black" AND "GLBT", or "black" and "woman" and which part of their self-identity should come first. The fight for self-identity is a human experience that is not isolated to people who exist in between two "distinct"(fictitious) groups. But when your identity is tied to "natural" (social) privilege or lack thereof, this fight simply becomes much more complicated.
It is very real in many societies that anything deemed vulnerable needs to be better protected. This ranges in all scopes of things from children to our immune systems, from white female virginity to our economic budget.
If European phenotypes are considered "naturally recessive" to all other "races" it essentially says that white genes are more valuable and therefore meant to be kept pure. Diamonds are only valuable because they are considered rare, otherwise, what would differentiate them from regular rocks?
We cannot separate racial perception from social conditioning. When we look at real life evidence, we can simply see that "race" doesn't exist.
Can we safely say now that the jig is up?
A-iOn