All 13 carriages of the train, which was travelling from Madrid to Ferrol, came off the tracks near the city of Santiago de Compostela.

Images from the scene showed bodies strewn near ruined carriages, and emergency crews searching the wreckage.

Analysts say it is the worst rail accident in Spain in four decades.

Leader of the regional government Alberto Nunez Feijoo confirmed the death toll but told radio Cadena Ser that it was too early to say what caused the accident.

"There are bodies lying on the railway track. It's a Dante-esque scene," he said in comments translated by the AFP news agency.

Prime Minister Mariano Rajoy convened an emergency ministerial meeting late on Wednesday and is due to visit the scene of the accident on Thursday.

"I want to express my affection and solidarity with the victims of the terrible train accident in Santiago," Mr Rajoy said on his Twitter account.

Local authorities have issued an appeal for residents to donate blood to hospitals in the area.

About 320 members of Spain's national police force have been deployed in response to the train derailment, officials say.

One witness, Ricardo Montesco, described how the train carriages "piled on top of one another" after the train hit a curve, speaking to Spanish Cadena Ser radio station.

"A lot of people were squashed on the bottom. We tried to squeeze out of the bottom of the wagons to get out and we realised the train was burning...I was in the second wagon and there was fire. I saw corpses," he added.

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The BBC's Tom Burridge in Madrid says Thursday is a local holiday in the region, and many people would have been preparing to celebrate.

However, festivities planned for Thursday have now been cancelled, local official Maria Pardo told Spanish television.

Local journalist Francisco Camino said the region was in shock.

"This is a tiny place and nothing happens here, nothing important or tragic," he told the BBC.

"We were preparing for the celebrations and now this could turn out to be the worst train crash in many years."

Railway firm Renfe said the train was carrying at least 218 passengers, and came off the tracks on a bend about 3km (two miles) from Santiago de Compostela station.

It was on the express route between Madrid and the ship-building city of Ferrol on the Galician coast.

Images showed dozens of emergency workers crowded around ruined carriages.

Passengers were shown lying on the ground being treated, and there were several bodies near the tracks.

Buildings collapsed during the attack in Sukkur

Gunmen burst into the heavily guarded complex in southern Sindh province and traded gunfire with security personnel.

At least five people were killed, most of them said to be attackers.

Buildings collapsed and officials fear people could be trapped in the debris. No group has said it carried out the attack, in the town of Sukkur.

Police said the target appeared to be the offices of the Inter-Services Intelligence agency (ISI).

Up to 10 attackers stormed the heavily guarded compound.

The gunmen seized control of one of the government buildings while another office was still under fire, according to the state-run Pakistan Television Corporation (PTV).

The roof of a third building was reported to have collapsed, with people feared trapped beneath the rubble.

Police sent reinforcements to the area, officials said, adding that it was not yet clear who was behind the assault.

There has been a series of attacks blamed on Islamist groups, including the Pakistan Taliban, around the country since the new Prime Minister Nawaz Sharif was sworn in last month.

Mr Sharif, who has pledged new action on terrorism, is under mounting pressure to explain how he intends stopping the bloodshed, observers say.

The US was expected to deliver the F-16 fighter jets in the next few weeks

Pentagon spokesman George Little said the decision was made "given the current situation in Egypt".

The US is examining whether Mr Morsi's removal was a coup, which would trigger a legal requirement to cut off aid.

Earlier, Egypt's army chief called for protests to give the military a mandate to confront "potential terrorism".

But Abdel Fattah al-Sisi said he was not calling for public unrest and he urged national reconciliation.

In response, the Muslim Brotherhood, which backs Mr Morsi, said Gen Sisi was "calling for civil war".

The four F-16 jets are part of an already agreed bigger order of 20 planes, eight of which have already been sent to Egypt.

As recently as 11 July US officials suggested the latest four F-16s would still be sent to Egypt.

However, on Wednesday the Pentagon confirmed that the delivery was on hold.

"Given the current situation in Egypt we do not believe it is appropriate to move forward at this time with the delivery of F-16s," Mr Little told reporters.

"We remain committed to the US-Egypt defence relationship as it remains a foundation of our broader strategic partnership with Egypt and serves as a pillar of regional stability," he said.

US Defence Secretary Chuck Hagel informed General Sisi of the decision in a phone call earlier on Wednesday, the Pentagon spokesman added.

The BBC's Kim Ghattas in Washington says the decision is a way for the US to signal its displeasure with recent events in Egypt without undermining Washington's strong relationship with the Egyptian military.

President Barack Obama took the decision late on Tuesday following the recommendation of his national security team, she adds.

US military aid to Egypt is estimated to be $1.3bn (£860m) each year.

Call for reconciliation

In a speech at a military graduation ceremony broadcast on television, Gen Sisi said: "I urge the people to take to the streets this coming Friday to prove their will and give me, the army and police, a mandate to confront possible violence and terrorism."

"So that in case there was a resort to violence and terrorism, the army would have a mandate to confront this."

Gen Sisi, who is also the defence minister in the new government, rejected rumours about divisions within the army ranks. "I swear by God that the Egyptian army is united," he said.

Referring to the army's intervention to remove Mr Morsi at the beginning of July, Gen Sisi said: "I urged the former president to be a president for all Egyptians".

The BBC's Quentin Sommerville in Cairo says Gen Sisi's call showed who was really in charge in Egypt - not the interim president picked by the military, Adly Mahmud Mansour, but the military itself.

He says it may be a sign of frustration that protests against the interim government are still going on.

Senior Brotherhood figure Mohamed el-Beltagy said Gen Sisi was "calling for a civil war... to protect this military coup".

"He is proving that he is the actual ruler of the country, and that the president, his vice president and the government do not hold any power," he added.

In Cairo, two people were reported to have been killed in clashes at a pro-Morsi demonstration overnight. That followed nine deaths in the city on Tuesday.

Mohammed Morsi narrowly won the presidential election in June 2012 to become Egypt's first democratically elected president, but his opponents accused him of trying to impose an Islamist agenda on the country.

Following weeks of often violent protests, the Egyptian military removed him from office and installed an interim government.

The company reported profits of $333m (£217m) in the second quarter.

"The work we've done to make mobile the best Facebook experience is showing good results and provides us with a solid foundation for the future," said founder Mark Zuckerberg in a statement.

Facebook is the world's largest social networking company.

According to chief financial officer David Ebersman, 20 billion minutes a day were spent on Facebook in June.

"This quarter marks the end of our first year as a public company and I think we've created a good foundation for the future," said Mr Zuckerberg on a call to discuss the earnings.

The company's share price is still down more than 16.7% from its stock market debut price of $38.

Mobile boost

Facebook saw $655.6m in revenue from mobile ads in the three months from April to June - more than 41% of the total $1.6bn it made from advertising overall, compared with 30% for the same period last year. The number of mobile users expanded 51% to 819 million.

Clark Fredricksen of research firm eMarketer told the BBC that the results were "incredibly impressive - they've exceeded expectations."

eMarketer estimates that Facebook will take in more than $2bn in mobile advertising this year and increase its share of the digital ad market to 5.04% - up from 4.11% in 2012.

"I'd say that Facebook is in a unique position with the ad market right now in that it is very far ahead of any other major ad platform in mobile display advertising," said Mr Fredricksen.

The company has focused on perfecting its ad sales programmes. Last month, it announced changes to its Ads Manager which it said would provide advertisers with better data.

In April, Facebook won back General Motors almost a year after the car company pulled advertising from the site.

Video boom

Mr Zuckerberg also discussed one of the company's most recent innovations: the introduction of video to Instagram, the photo sharing platform that Facebook bought in April 2012 for $1bn.

The video feature was introduced to compete with Twitter's short video sharing service, Vine.

"Adding video fits really naturally with the Instagram mission," said Mr Zuckerberg.

On the call, analysts asked a question that many have wondered: When will Instagram finally begin to generate any sort of revenue for the company?

Mr Zuckerberg, while emphasising that it's "been clear that we're building Instagram to be a business", simply said that Facebook was focused on building Instagram's "imprint", or user base.

He also alluded to two new products that have yet to gain traction with Facebook users: Home, the company's mobile phone software, and Graph Search.

Mr Zuckerberg said he thought these products "will take longer to develop."

The revival in the eurozone economy is being led by manufacturing

The Markit eurozone Purchasing Managers' Index (PMI), which measures business output, was 50.4 in July. A figure above 50 indicates expansion.

July's figure was up from 48.7 in June, and marks an 18-month high.

The eurozone has been in recession since the end of 2011.

"The best PMI reading for one-and-a-half years provides encouraging evidence to suggest that the euro area could - at long last - pull out of its recession in the third quarter," said Chris Williamson, chief economist at Markit.

He said the revival in the economy was being led by a broad-based upturn in manufacturing, and signs of stabilisation in the services sector.

But he warned that employment was continuing to fall, although at a slower rate than earlier in the year.

Markit's survey found that manufacturers reported the largest monthly increase in output since June 2011, and output in the sector grew for the first time since February 2012.

Service sector activity fell, but the decline was the smallest in 18 months, and Markit said the data showed signs of stabilising after "marked rates of decline" earlier in the year.

The PMI measure is based on surveys of thousands of companies across the eurozone and is regarded as a reliable indicator of economic growth.

Mr Williamson said the latest data tentatively suggested the eurozone would grow by 0.1% in the third quarter of this year.

The latest forecasts from the European Central Bank are for the eurozone economy to contract by 0.6% over 2013 as a whole, but for it to recover with growth of 1.1% in 2014.

The data revealed a continuing gap between eurozone members.

Germany continued to perform well, with steep growth in manufacturing and services, and strong job creation.

However, southern European states saw output falling, but only marginally.

However, the pharmaceutical giant said it was "too early to quantify" the extent of the scandal.

GSK has been accused by Chinese authorities of directing up to £320m through travel agencies to facilitate bribes to doctors and officials.

On Monday, GSK said senior executives in its China office appeared to have broken the law.

Several GSK employees have been detained over the last few weeks and the firm's British head of finance in China, Steve Nechelput, has been subject to a travel ban since the end of June.

GSK's chief executive, Sir Andrew Witty, said that it appeared the investigation was centred on a number of senior executives in China who were allegedly operating outside of the company's systems and processes.

He described the situation as "shameful" and "deeply disappointing".

"We have zero tolerance for this behaviour and are committed to rooting out corruption," he said.

"Unfortunately, there is a risk that some individuals can operate outside our systems and controls," he said, adding, "99.99% of the people in GSK operate within the rules."

Sir Andrew said that GSK remained "fully committed" to China and that the company had "many important medicines and vaccines which will be important to the Chinese people".

He added that it was too early in the investigation process to tell whether or not criminal charges might be laid against the company itself by the Chinese authorities.

BBC business editor Robert Peston tweeted: "Disappointing press conference with Sir Andrew Witty on GSK's China scandal. He said less than already in public domain, and rowed back from admission of guilt."

Meanwhile, GSK announced a 2% rise in second-quarter group sales, to £6.62bn ($10.17bn).

The company has been struggling of late owing to loss of patent protection on some of its medicines and falling prices in austerity-hit Europe.

The boat sank off the Indonesian island of Java, the transit point for people-smugglers.

At least 157 people have been rescued. It is not clear how many are missing.

Meanwhile, Australia's immigration minister said he would investigate abuse claims at the country's offshore processing centre in Papua New Guinea.

Prime Minister Kevin Rudd announced a new asylum policy last week, ahead of polls expected to be announced soon.

Under the policy, asylum-seekers arriving by boat in Australia will be sent to Papua New Guinea (PNG) for processing, and those whose refugee claims are upheld will be settled in PNG, rather than Australia.

Australia has experienced a sharp increase in the number of asylum seekers arriving by boat in recent months. But critics have accused Australia of avoiding responsibility and passing on its problem to a developing nation.

Mr Rudd said that the sinking underlined the need for a policy shift, saying the government had to send "a very clear message to people-smugglers to stop sending people by boat to Australia".

"We are seeing too many drownings, we are seeing too many sinkings, too many innocent people being lost at sea."

PNG is to receive Australian investment as part of the deal. But some PNG politicians say the agreement could cause tensions on the island.

Opposition spokesman Tobias Kulang said PNG had "become a dumping ground for Australia's inadequacies".

"This is an appalling performance by Australia, which with its monetary wealth is able to pass the buck on to poorer countries," he said.

'Helpless and hopeless'

The latest sinking, which happened on Tuesday night, involved passengers who said they were from Iraq, Iran, Sri Lanka and Syria.

At least two children are among the dead. More people are feared missing but officials still do not know the exact number of people who were on board.

Fishermen first spotted the asylum seekers - men, women and children - swimming to the shore near the West Java town of Cidaun on Tuesday evening, the BBC's Alice Budisatrijo in Jakarta reports.

The head of the local rescue agency told the BBC they were now being held in a nearby immigration facility.

Another boat carrying around 38 asylum seekers has also been stopped near Christmas Island, reports say.

Many asylum seekers seek to journey to Christmas Island, which is the closest part of Australia to Indonesia and lies 1,600 miles north-west of mainland Australia.

Meanwhile, Immigration Minister Tony Burke said he would travel to Manus Island, Australia's offshore processing centre in PNG, after allegations of abuse there emerged.

A former security manager, Rod St George, told Australia's Special Broadcasting Service that some detainees had been raped and assaulted.

He said the the facilities at the site were not "even fit to be used as a dog kennel".

Mr Burke has described the allegations as "horrific", and said that he intended to "work through" issues at the island.

Asylum policy is expected to be a key issue in Australia's elections, which must take place by 30 November.

An opinion poll on Tuesday suggested that the opposition coalition led Mr Rudd's Labor party by 52% to 48% after preferences.

However, Mr Rudd is still polled as voters' preferred prime minister, at 50%, compared to opposition leader Tony Abbott's 34%.

Mr Rudd ousted Julia Gillard as Labor Party leader last month, amid dismal pre-election polling figures.

Grieving relatives are angry at the school authorities

Meena Kumari had been sought since the tragedy in eastern Bihar state. She handed herself in on Wednesday.

Police believe she can help explain how the deadly insecticide monocrotophos ended up in the free school lunch.

In all, 47 primary school children fell ill after consuming the meal of rice and soybeans.

The Mid-Day meal scheme, introduced to combat hunger and boost school attendance, provides free food for 120 million children in 1.2 million schools across India.

It often suffers from poor hygiene, and poisoning outbreaks occur from time to time, although rarely if ever with such deadly results.

All of the teachers in Bihar's state-run primary schools have vowed not to participate in the scheme from Friday.

"Teachers and the headmasters have to manage the scheme with little help, low-quality food and corruption at every level," said B Sharma, head of the primary school teachers' association.

"But when such an incident happens, a teacher or the principal is held responsible."

Earlier, police said scientific tests had detected "very toxic" levels of the pesticide monocrotophos in the deadly meals.

Vegetable oil used to prepare the food was revealed to be highly contaminated.

"It was the high quantity of monocrotophos insecticide found in the food which proved fatal for the schoolchildren," said Ravindra Kumar, a top police official in Bihar state capital Patna.

The school's cook, Manju Devi, had earlier accused the school principal, Meena Kumari, of forcing her to use the oil, despite the cook complaining that it "smelled strongly" and looked "dodgy". She said she was told the oil was "home-made and safe to use".

State Education Minister, PK Shahi, said it had been bought from the grocery store run by the teacher's husband.

Police have also been investigating whether the oil was kept in a container previously used to store pesticide.

Reports say Meena Kumari was on her way to the court to hand herself in on Wednesday, but officers stopped her and arrested her before she could go inside.

She faces charges of murder and gross negligence, according to police.

"We have arrested her for questioning," said Sujeet Kumar, the police chief of Saran district, where the children died.

He told the AFP news agency "we need to talk to her first" before bringing charges.

Bihar Chief Minister Nitish Kumar and other state officials have been criticised for being slow to respond to the tragedy.

"I assure you, those found guilty will be taken to court and punished," he told a news conference shortly after the arrest.

Locals complained that it took days for any top-level official to visit the village of Dharmasati Gandaman, where the poisoning occurred on 16 July.

There have been widespread protests following the deaths and children across India have reportedly been refusing school meals.

GROS ISLET: After ensuring that their unbeaten run in head-to-head series in the West Indies since the 1988 tour will be extended, Pakistan are in no mood to relax when the final One-Day International of the five-match series against West Indies is played at the Beausejour Stadium in St Lucia on Wednesday.

Following a six-wicket win in the rain-affected fourth match at the same venue on Sunday, the tourists enjoy an unbeatable 2-1 lead in the series and the hosts are under increasing pressure to get the win that will square the rubber and at least reassure Caribbean fans that the regional team is steadily improving in the countdown to the next World Cup.

The third fixture, also played at Beausejour, finished in a tie.

“We're looking forward to winning again. We gained a lot from that win (on Sunday) and confidence will be high for this final game,” said Pakistan captain Misbah-ul-Haq in anticipating the final challenge.

He was full of praise for left-arm seamer Junaid Khan. Together with Mohammad Irfan, they have been the most effective spearheads for Pakistan.

However their third left-armer, Wahab Riaz, has found the going tough in the last two matches and may be forced to make way for Asad Ali, who opened the bowling in the first two matches in Guyana.

Despite proving to be extremely expensive in the final overs of their opponents' innings so far in the series, West Indies captain Dwayne Bravo has continued to opt for his medium-pacers at the death, often resulting in a significant acceleration of the Pakistani scoring.

He finally admitted in the aftermath of defeat on Sunday that those tactics will have to be revisited.

Yet it remains to be seen what he will choose to do on the day given the reluctance to utilise Darren Sammy when the late charge is on, despite the fact that the former ODI captain has been consistently one of the most economical bowlers with his medium-pacers.

Following Wednesday's series finale, the teams travel to St Vincent for back-to-back T20 Internationals at the weekend, which will conclude Pakistan's short tour.

KIRKUK: Bombs targeting worshippers gathered for evening prayers at four Sunni mosques in Iraq killed 12 people on Tuesday, officials said.

A bomb exploded in the Omar bin Abdulaziz Mosqe in the northern city of Kiruk, while another blast struck near Al-Salihin Mosque close by, killing a total of seven people and wounding 31.

Two bombs exploded near Ahmed al-Mukhtar Mosque in the Dura area of south Baghdad, killing at least three people and wounding at least nine.

And a car bomb exploded near Al-Imam Ali Mosque in Kut, southeast of Baghdad, killing two people and wounding nine.

Both Sunni and Shia places of worship have been attacked in recent months, raising fears of a return to the all-out sectarian conflict that killed tens of thousands of people in past years.

With the latest attacks, more than 630 people have been killed in violence so far this month, making July the deadliest month in a year marked by spiraling violence.

Months after promising peace talks with insurgents, Pakistan's new prime minister appears to be backing down and accepting that the use of military force may be unavoidable in the face of escalating violence across the South Asian country.

Almost 200 people have been killed in rebel attacks in Pakistan since Prime Minister Nawaz Sharif came to power last month, advocating peace talks with the Pakistani branch of the Taliban.

Sharif's tougher line signals that Pakistan's powerful military still has the upper hand in policy-making, despite hopes that the government would have a larger say after he came to power in the country's first transition between civilian administrations.

"Of course we want to try talks but they are a far off possibility," said a government official, who has knowledge of discussions between civilian and military leaders on how to tackle the Taliban.

"There is so much ground work that needs to be done. And when you are dealing with a group as diverse and internally divided as the Pakistani Taliban, then you can never be sure that every sub-group would honour talks."

The military has ruled Pakistan for more than half the 66 years it has been independent.

Seeking to dispel a view that he is losing the momentum, Sharif, who once said that "guns and bullets are not always the answer", has promised to come up with a new security strategy.

But progress has been painfully slow, blighted by infighting and the army's long-standing contempt for the civilian leadership.

An official report into the killing of Osama bin Laden by US forces in Pakistan in 2011, leaked this month, offered striking insights into just how deep this distrust runs.

In the document, the former chief of the ISI intelligence agency, which is dominated by the military, was quoted as saying bluntly that the country's political leadership was "unable to formulate any policy".

In the meantime, attacks continue unabated.

A bomb ripped through a busy street in Lahore on July 7, striking in the heart of Sharif's otherwise relatively peaceful home city. President Asif Ali Zardari's security chief was killed in a suicide bomb in Karachi on July 10.

"They (the Pakistani Taliban) see this as an opportunity. They want to send a message to Nawaz Sharif of their strength and his relative weakness," said Ahmed Rashid, an author and expert on the Taliban.

"The army is against the talks right now. They want to hammer these guys a little bit more."

Yet, the military and the ISI are in favour of talks involving the Taliban in neighbouring Afghanistan. Although the Pakistani Taliban accepts the leader of the Afghan faction as its own leader, the two groups operate separately.

ISLAMABAD: The stage will be set for next month’s presidential election on Wednesday, with the deadline for submission of nomination papers for the top slot expiring at 12 noon.

Pakistan Muslim League-Nawaz (PML-N) has officially nominated Mamnoon Hussain for the post of presidential candidate. Later, he submitted his nomination papers in the Islamabad High Court (IHC).

Pakistan Peoples Party's candidate Senator Raza Rabbani has also submitted his nomination papers. Earlier, Awami National Party (ANP) and BNP-Awami have declared senior PPP leader Senator Raza Rabbani as their joint presidential candidate.

The Pakistan Tehrik-i-Insaf (PTI) candidate Justice Wajhiuddin has also submitted his nomination papers in the Sindh High Court (SHC).

According to DawnNews, the Muttahida Qaumi Movement (MQM) decided not to contest for the elections and will be supporting another party's candidate for the post of president.

As per the schedule, the scrutiny process will take place on July 26. The candidates will have the option of withdrawal of nomination papers on July 27 and the final list of candidates will also be issued on the same day. Polling for the presidential office is scheduled to be held on July 30.

Senior PML-N leaders had earlier told Dawn that the former governor of Sindh was the front-runner among the party’s shortlisted candidates, followed by PM’s adviser on foreign affairs and national security Sartaj Aziz and former chief justice of Pakistan Justice Saeed-uz-Zaman Siddiqui.

Keeping in view the position of the electoral college, the ruling party’s nominee is expected to emerge as the winner.

Meanwhile, the Supreme Court on Wednesday ordered for the presidential election to be held on July 30, following the decision of the Election Commission of Pakistan (ECP) that it would not give a new date for the polls.

ISLAMABAD: The Supreme Court on Wednesday ordered for the presidential election to be held on July 30, following the decision of the Election Commission of Pakistan (ECP) that it would not give a new date for the polls.

Chief Justice Iftikhar Muhammad Chaudhry also instructed Director General (DG) of the election commission to execute the court's order and to complete the scrutiny process by tomorrow.

Earlier, the ECP announced that it was leaving the matter in the hands of the SC.

The commission's view came after the SC gave it until 11 am to convey its stance on an application submitted by Senator Raja Zafarul Haq of the ruling Pakistan Muslim League – Nawaz (PML-N) seeking to hold the upcoming presidential election before the announced date of Aug 6.

Haq had submitted his petition in the SC after the ECP had turned down a request made by the federal government to change the date for presidential election, saying that polls would be held on Aug 6 as announced.

Earlier, a three-member of the apex court, headed by Chief Justice Iftikhar, heard Zafarul Haq’s petition.

The PML-N senator, in his request, asked the apex court not to hold the presidential election during the last Ashra (ten days) of Ramazan.

He further gave the reference of various reports published in the media, according to which more than a hundred lawmakers would not be available on Aug 6 to participate in the presidential election, as they would be engaged in Aitekaf, Umrah and other religious duties.

Haq's petition sought to hold the presidential election on July 30.

During the hearing, Attorney General Munir A. Malik informed the court that the federal government supported Haq’s petition. Moreover, he said it was the constitutional right of every member of the Parliament to vote in the presidential polls.

Chief Justice Iftikhar asked as to what would be the constitutional barriers in preponing the election, adding that the participation of every parliamentarian in the polls would permit constitutional requirements to be better fulfilled.

He remarked that the ECP should consider holding the election on the date requested by Haq.

Extroverts, those outgoing, gregarious types who wear their personalities on their sleeve, are generally happier, studies show. Some research also has found that introverts, who are more withdrawn in nature, will feel a greater sense of happiness if they act extroverted.

Experts aren't entirely sure why behaving like an extrovert makes people feel better. One theory is that being talkative and engaging influences how people respond to you, especially if that response is positive. Others speculate that people get more satisfaction when they express their core self and opinions. Another possibility: Happiness might come simply from having successfully completed a goal, such as giving a speech.

"If you're introverted and act extroverted, you will be happier. It doesn't matter who you are, it's all about what you do," said William Fleeson, a psychology professor at Wake Forest University in Winston-Salem, N.C.

Other studies have shown that extroverts are more motivated than introverts. Researchers believe this is due in part to extroverts' greater sensitivity to dopamine, a neurotransmitter that plays a big role in behavior driven by rewards.

Clark Powell considers himself an introvert, but his job as vice president of a media relations and multimedia production firm means he often must act out of character. "My job requires me to be on TV…and to do media training and presentations in front of large groups of people, as painful as that can be sometimes," said the 46-year-old in Columbia, Ohio. Though he may dread making a presentation, he says he is exhilarated afterward. "I do feel a sense of relief and elation, but I don't know if that's because of the experience or because the experience is over," he said.

Mr. Powell disagrees with research findings that extroverts are happier and more motivated. While extroverts might derive happiness through feedback from others, Mr. Powell says his sources of pleasure include learning new things and reading a good book. "I may not share my happiness as willingly as other people…but I consider myself just as happy and I'm extremely motivated to learn and grow as an individual."

Whether a person is an extrovert or introvert is one of the big five traits commonly used by psychologists to classify personalities. (The others are openness to experience, conscientiousness, agreeableness and neuroticism.) Researchers say people generally fall somewhere in the middle, with attributes of both types. Extroverts tend to thrive off of interaction with other people. Introverts are typically more reserved, but not necessarily shy. They prefer solitary behavior or engaging in small groups.

Researchers say genetics may play a large role in whether we are more extroverted or introverted. Social experiences, especially those outside of the family environment, are also important, particularly as a child and young adult when the connectivity between neurons is being established.

Dr. Fleeson, of Wake Forest University, reported in a 2012 article in the Journal of Personality the results of an experiment that found introverts experience greater levels of happiness when they act more extroverted. In the weeklong study, researchers followed 85 people who recorded on Palm Pilots how extroverted they were acting and how happy they were feeling. Other studies of introvert behavior have reached similar conclusions.

So why don't introverts act like extroverts more often? John Zelenski, a psychologist at Carleton University in Ottawa, and fellow researchers probed that question in an April article in the Journal of Personality and Social Psychology.

A series of studies, which included more than 600 college students, found that introverts misjudge how they would feel after acting extroverted. They often predicted feelings of anxiety and embarrassment, which never transpired.

"Introverts kind of underestimate how much fun it will be to act extroverted," said Dr. Zelenski. "You don't think you want to go to a party and then go and have a great time." Dr. Zelenski and other researchers also considered whether people acting in a way that goes against their natural disposition might wear themselves out. In two studies, a total of about 150 college students were instructed to behave in an extroverted or introverted manner during a group activity. Questionnaires and cognitive tests measured how much mental energy was depleted.

"We didn't find a lot of evidence for…the idea that acting like an extrovert would wear out introverts," said Dr. Zelenski. However, he said: "We found acting like an introvert tended to wear out extroverts," who performed worse on cognitive tests.

Still, Brian Little a psychology professor at Cambridge University in the United Kingdom, believes that acting out of character can take a physiological toll on the nervous and immune systems. Dr. Little says he's an introvert who often has to engage in extroverted behavior, such as making speeches, in order to advance his work. Afterward, Dr. Little says he often needs to emotionally recharge.

While extroverts might benefit from interacting with others during a break at a conference, Dr. Little prefers to go for walk if he has time. If not, he might hide in the restroom. "As an introvert acting as an extrovert I need to escape from the vicissitudes of overstimulating colleagues," he said.

Dr. Little says some of his students are starting a study to explore the cost of acting out of character. "I'm quite confident that we can show that going against your traits is going to use up resources," such as glucose, he said. "Anything that requires concentration is going to deplete glucose resources," he said.

Researchers say genetic differences also can account for why introverts don't seek to act extroverted. That's because introverts don't get the payoff for that behavior that extroverts do, in the form of heightened sensitivity to the neurotransmitter dopamine. The reasons this occurs aren't fully understood, says Richard Depue, a professor of neuroscience in the department of human development at Cornell University. The bottom line: Extroverts are wired to act more motivated to get that reward.

Luke Smillie, a senior lecturer of psychology at the University of Melbourne in Australia, notes that most studies of introverts and extroverts take place in the U.S. and other western countries where extroversion is often perceived to be more valuable. "The question is, would you observe the same effects in cultures that didn't have this sort of value placed on being outgoing and assertive and so forth?" he said.

"We live in a culture that very much subscribes to the extrovert ideal of being bold and assertive," said Susan Cain, a former corporate lawyer who wrote a book last year called "Quiet: The Power of Introverts in a World That Can't Stop Talking," which argues that introverts are unfairly maligned. Rather than trying to get introverts to act more extroverted, she argues that society should be drawing on their natural strengths, which can include being a good listener and working creatively.