Possible Theories

Interestingly enough, the theories made about the Lisbon girls tend to reflect more on their theorists then the girls themselves:

"Platelet serotonin receptor indices in suicidal children" (Eugenides 286)
The doctor assigned to the Lisbon case determined that Mary had low serotonin levels. I find that often professional adults prefer to use science to explain phenomenons, instead of digging deeper to the root emotional cause. Often professional adults are uncomfortable with the idea of teenagers having real tangible emotions that can end in tragedy. I often feel as if they don't want to give young adults that power; the prefer to devalue their emotions, writing them off as being "just a teenager".

"The suicides  were an esoteric ritual of self sacrifice" (Eugenides 289)
"The girls planned the suicides in concert with an undetermined astrological event" (Eugenides 289)
This is theorized by the persistant local news reporter Lydia Perle. Like most adults, she wanted someone to blame, without pointing her finger at herself or people like her. It is highly unlikely that the Lisbon sisters were involved in any sort of ritual self-sacrifice. However, it does direct the blame somewhere else by distracting the community, and like any other reporter, Perle just wants to find a story that will sell to the community. It is sad how adults tend to write off things they can't understand, or events that might leave them with some degree of blame.

"They reacted to the suicides with a mild shock, as though they'd been expecting them, or something worse" (Eugenides 300).
The apathy of adults in this community is astonishing and borderline sickening.

"Something sick at the heart of the country infected the girls" (Eugenides 301)
"People saw their  clairvoyance in the wiped-out elms, the harsh sunlight, the continuing decline of our auto industry" (Eugenides 318)
This theory might have some weight to it. In a way, I think the girls were overwhelmed with the ways of the world they lived in. Eugenides mentions Detroit several times in his explanation of this theory. The way the country, more specifically the city was being run did not work for the girls, and they felt powerless, like many people did at the time to change it. In the society, not enough merit was placed on thoughts and feelings of young girls. They felt left behind and unneeded, in a world that was progressing too fast for them, but leaving so many holes behind.

"They killed themselves over our dying forests, over manatees maimed by propellers as they surfaced to drink from garden hoses; they had killed themselves at the sight of used tires stacked higher than pyramids; they had killed themselves over the failure to find a love none of us could ever be. In the end, the torture tearing the girls pointed to a simple reasoned refusal to accept the world as it was handed down to them, so full of flaws" (Eugenides 320)
I think this is the single most important explanation as to why the Lisbon girls decided to end their lives. They were too good for the world that they lived in, and they could not bear to go through with the long lives ahead of them knowing the type of evil and sadness that frequently occurs on Earth. Cecilia knew all about the unfairness; she wrote in her diary about the endangered species list. The rest of the girls got a taste for the unfairness of the world when their sister died, their parents ignored them and they were alienated at school and by society. Lux got a taste of the unfairness in the world when she met Trip. Isolated and treated as a museum exhibit, I can understand why they would want to be with their departed sister then inside their mother's house.

"So much has been written about the girls in the newspapers, so much has been said over back-yard fences, or related over the years in psychiatrists' offices, that we are certain only of the insufficiency of the explanations" (Eugenides 323)
Eugenides stresses that there is no definite answer. The boys don't know, the media doesn't know. The parents, teachers, former friends, family, doctors and even the girls themselves may not even know why the Lisbon girls all died within a year. There was no note left, no clues, no hidden diaries or telephone calls. The story revolves around the death of youth and beauty, and sometimes youth and beauty die without entrusting an explanation.