Claudius conducts a census in 48 that finds 5,984,072 Roman citizens, an increase of around a million since the census conducted at Augustus' death.

He has helped increase this number through the foundation of Roman colonies that were granted blanket citizenship.

These colonies are often made out of existing communities, especially those with elites who could rally the populace to the Roman cause.

Several colonies are placed in new provinces or on the border of the Empire in order to secure Roman holdings as quickly as possible.

Some years after divorcing Aelia Paetina, in 38 or early 39, Claudius had married Valeria Messalina, who was his first cousin once removed and closely allied with Caligula's circle.

The ancient historians allege that Messalina was a nymphomaniac who was regularly unfaithful to Claudius—Tacitus states she went so far as to compete with a prostitute to see who could have the most sexual partners in a night—and manipulated his policies in order to amass wealth.

Messalina in 48 marries her lover Gaius Silius in a public ceremony while Claudius is at Ostia.

Sources disagree as to whether or not she divorced the Emperor first, and whether the intention was to usurp the throne.

Vincent Scramuzza, in his biography (The Emperor Claudius Cambridge: Harvard University Press [1940]), suggests that Silius may have persuaded Messalina that Claudius was doomed, and the union was her only hope of retaining rank and protecting her children.

The historian Tacitus suggests that Claudius's ongoing term as Censor may have prevented him from noticing the affair before it reached such a critical point.

Whatever the case, the result is the execution of Silius, Messalina, and most of her circle.

After the downfall of his mother, Britannicus' youth becomes a liability for Claudius.

The lack of an adult heir makes the emperor vulnerable to conspiracies aimed at overthrowing the dynasty, especially those by other Julio-Claudians.

Around this time, his niece Agrippina the Younger becomes the mistress to one of Claudius’ advisers, the former Greek Freedman Marcus Antonius Pallas.

At this time, Claudius’ advisers are discussing which noblewoman Claudius should marry.

Claudius has a reputation that he is easily controlled by his wives and freedmen.

His freedmen, according to legend, presents him three possible candidates.

The freedman Tiberius Claudius Narcissus suggests Claudius remarry his second wife Aelia Paetina, with whom he has a daughter, Claudia Antonia.

Narcissus also states that Paetina will cherish Claudia Octavia and Britannicus, Claudius's children with Messalina.

Another freedman, Gaius Julius Callistus, is against Claudius remarrying Paetina and states to Claudius that he divorced her before and that remarrying Paetina would make her more arrogant.

Callistus suggests Lollia Paulina, Caligula's third wife and Agrippina's former sister-in-law instead.

Pallas advises Claudius that he should marry Agrippina, stating to the emperor that as Lucius was the grandson to Claudius's late brother Germanicus, by marrying Agrippina, Claudius would ally the two branches of the Claudian house and imperial family.

In more recent times, it has been suggested that the Senate may have pushed for the marriage between Agrippina and Claudius to end the feud between the Julian and Claudian branches.

This feud dates back to Agrippina's mother's actions against Tiberius after the death of Germanicus, actions that Tiberius had gladly punished.

Regardless, for Agrippina’s seduction, it is a help that she has the niece’s privilege of kissing and caressing her paternal uncle.

Seduced by her passions, Claudius makes references to her in his speeches: "my daughter and foster child, born and bred, in my lap, so to speak".

When Claudius decides to marry her, he persuades a group of senators that the marriage should be arranged in the public interest.

In Roman society, an uncle (Claudius) marrying his niece (Agrippina) is considered incestuous, and obviously immoral.