Were her parents negligent to leave Madeleine and her 2-year-old twin siblings alone, even for a brief time? Or should they have been able to expect a certain level of safety in this family-friendly southern resort town?
The questions cut to the heart of a universal vulnerability that terrifies parents everywhere. Yet answers differ widely — revealing cultural differences within Europe and across the Atlantic.
In Portugal and much of the rest of southern Europe, where parents often take their young children along with them to smoky bars, many have accused the McCanns of neglect, despite the fact that they were at a poolside restaurant just seconds away from the room and say they checked on their sleeping children every half-hour. The resort offers baby-sitting services, but the McCanns apparently chose not to use them that evening.
"You shouldn't leave (young children) alone," said Francisco Vieira, a 77-year-old father of two grown children who works as a parking lot attendant near the beach in Praia da Luz.
He said an abduction is "not the kind of thing you'd expect here, but you still shouldn't risk it. We never left our children alone. We'd either take them with us or one of us would stay behind."
Many Portuguese travelers express distaste over attitudes toward children in Britain, particularly notices in some British pubs that make it clear children are not welcome.
British parents, and many of their American counterparts, object to the second-hand smoke and loud music to which kids are subjected on nights out in Spain and Portugal.
Culture clashes have emerged as a theme in child dramas in the past.
In 1997, a toddler was left in a stroller outside a New York City restaurant while her mother dined and drank inside, prompting diners to phone police and complain.
After being arrested for neglect, the mother maintained the practice was common in her native Denmark and sued the city for false arrest. The charges were dropped and she was awarded $66,401.
Six years later, a Swiss father was arrested on charges of child endangerment after a maid discovered his 2-month-old in the family's Waldorf-Astoria hotel room while he and his wife were out to lunch.
"We are nice people and good parents. It's just a matter of a different culture," the wife told The New York Post, calling it a simple mistake. Here, too, charges were eventually dismissed.
Sharman Stein, director of communications at New York City's Administration for Children's Services, said her agency would definitely conduct an investigation if they received a call that children as young as 4 were left unsupervised.
While Stein said laws often don't specify the age at which it's safe to leave children alone, she said she didn't know any child protection expert who would consider 4 an acceptable age. She said the professional wisdom is generally that children should not be left alone before the "age of reason," which psychologists put at about 7 or 8 years old.
"Children 4 and under should not be left alone," she said, noting that Children's Services has a campaign called "Take Good Care of Your Baby" that urges parents to never, under any circumstances, no matter how briefly, leave their children alone. "It's not good parenting. It's just not safe," she said.
Even mild criticism of the McCanns, both doctors from central England, has caused outrage in Britain, where the nation has rallied around the family and the case has become a media phenomenon.
Images of the blonde little girl are ubiquitous on TV and in print, and the issue dominated radio and TV shows for two weeks until the decision not to send Prince Harry to Iraq briefly eclipsed the McCann saga.
Celebrities such as J.K. Rowling, author of the Harry Potter books, and entrepreneur Richard Branson have pledged money to a reward fund that now tops $4.9 million.
The McCanns said in a statement they were "totally overwhelmed" by the response to an Internet-based campaign they launched Wednesday in Britain.
Their Web site had already received 5 million hits, they said.
The British tabloids, which have sent packs of newshounds to Praia da Luz, have responded to Portuguese criticism of the family by taking aim at local police, complaining about lack of information and perceived lack of progress in the case.
They have taken to calling the police chief "hapless" and accused other officers of being asleep on the job.
Friends of the McCanns have described them as doting, overprotective parents, insisting there is no way they could have imagined their child would be snatched from the resort compound while they sat so nearby.
Portuguese police have questioned the parents extensively as witnesses in the May 3 disappearance, but they have not been named as suspects in any crime. A 33-year-old British man who lives near the hotel has been questioned as the only suspect but was released for lack of evidence.
Jon Clarke, 34, a physics teacher in London, said parents in Britain are not encouraged to take children to restaurants, and that he would consider leaving his own 3-year-old alone if it was in a safe place where he could easily check on her.
"If you take children to a restaurant in Britain, it's more often the attitude that the children shouldn't be there, whereas in Spain, Italy, France, they're more welcoming," he said.
In Spain, which is famously child-friendly, what the McCanns did is all but unheard of. Spanish parents take their kids everywhere, and it is common to see small children running around a town square while parents have drinks well into the night.
"People just say, 'Oh well, they'll sleep late tomorrow,'" said Ines Alberdi, a professor of sociology and family issues at Complutense University in Madrid.
Spaniards, she said, "do not totally separate children's entertainment from parents' entertainment. I think that is a very strong tradition here."
Magda Carlan, a 37-year-old Portuguese housewife with daughters aged 2 and 4, blamed the McCanns for their own nightmare. "Children should never be left alone. It is wrong. When I go on vacation with small girls, I am very careful."
The media frenzy has whipped up a certain hysteria among many parents in Britain, resulting in a surge in interest in electronic tracking devices that could potentially help police find missing children.
Richard Howells, a professor of cultural and creative industries at King's College in London, said the case has touched a nerve in Britain because it is so easy to identify with the family.
Many Britons vacation in the Algarve region of Portugal, where Praia da Luz is located, drawn by its reputation as an affordable family holiday destination.
"You can project yourself in that situation and you can feel how they're feeling, how terrible it is," he said. "Even though you don't personally know them, you feel through the media that you can."