By default, is DispatchQueue .serial or .concurrent?

Prior to Swift 3, the default dispatch queue type was serial – passing nil into the attributes parameter of dispatch_queue_create would yield a serial queue, and I see no reason for the default queue type to change. Although unfortunately I can't find any documentation on DispatchQueue that can confirm this.

However, looking at the source code reveals that this is indeed still the case:

public convenience init(
    label: String,
    attributes: DispatchQueueAttributes = .serial, 
    target: DispatchQueue? = nil)
{
    ...
}

Although I always prefer to specify the attribute explicitly, to make my code clearer and prevent this kind of confusion.

UPD Swift 5

The answer by @Hamish holds true: a new DispatchQueue is serial by default. And though the default value for attributes in the source code does not specify ".serial" anymore as it used to:

public convenience init(
        label: String,
        qos: DispatchQoS = .unspecified,
        attributes: Attributes = [],
        autoreleaseFrequency: AutoreleaseFrequency = .inherit,
        target: DispatchQueue? = nil)

it is noteworthy that the set of Attribute values has .concurrent option but not .serial any more. So again, serial by default, and concurrent if otherwise explicitly assigned in the attributes array.