"It was produced prior to March 2001" and relied on "Citizenship laws are based on the Nationality Act of 1965 with Amendment No.2 AD 1992 and Amendment No.3 AD 1993."
A bit of further research, and I'm surprised just how little information is searchable in English.The Act gives a child, born to a Thai and foreign parent, the opportunity between ages 20 and 21 of renouncing Thai citizenship,
if they so wish. You may also chose to note that there is no penalty for not choosing - after which they can continue to hold dual citizenship.
Section 14
A person of Thai nationality, who was born of an alien father or mother and has acquired the nationality of the father or mother according to the law on nationality of the father or mother, or a person who acquired Thai nationality under Section 12 paragraph two or Section 12/1 (2) and (3) is required, if he desires to retain his other nationality, to make a declaration of his intention to renounce his Thai nationality within one year after his attaining the age of 20 years, according to such forms and in the manner as prescribed in the Ministerial Regulations.
Context is important here - the second version of the Nationality Act in 1992 DID have a penalty for people not
choosing (ie automatically forfeiting of Thai Citizenship if a choice was not made) but that was done away with in Version 3 of the Act, released later in 1992.
Subsequent versions of the act have kept that language exactly the same, except for an amendment in 2008 making this clause apply to those both for a foreign father or mother.
Gender equity and all that...
Later ...
Q: Does Thailand allow dual nationality?
Does Thailand allow its citizens to have dual nationality? For instance, can a Thai national keep her Thai citizenship and acquire a foreign one? If so, can that person then have two passports?
Answer 1: “Yes, Thai citizens are allowed to have dual nationality and own two passports. If you are born in Thailand and have at least one Thai parent, you automatically have Thai nationality and can apply for a Thai passport.
However, if you are only half-Thai and were born and raised aboard, and do not have Thai nationality, you can still get a Thai passport, but must contact the District Office that your Thai parent is registered at and have your name added to the Thai House Registration list.
For more information contact the Phuket Passport Office Monday to Friday 8:30am to 4:30pm at 076-222 081.”
Thursday, October 11, 2012 12:20:07 PM
Worrawan Pakamas, Phuket Passport Officer responsible for checking nationality and identity.
Answer 2: “Although you are allowed to own two passports, you must leave Thailand on the same passport you used to enter The Kingdom.”
Thursday, October 11, 2012 12:20:07 PM
Lt Col Napat Nusen, Head Supervisor of the Phuket Immigration office.
source:
Issues & Answers
Dual nationality for Thais
Question: If a Thai woman marries a man from the US and she later becomes a US citizen, does the Thai woman lose her Thai citizenship?
Answer: “Section 13 of the Citizenship Act 1965 stipulates that a female Thai national who marries a foreigner and is later granted citizenship by her foreign husband’s home country shall maintain her Thai nationality.
There is no Thai law that stipulates that she must lose her Thai citizenship. However, she may choose to revoke her Thai citizenship, a decision the Thai government will announce the
Government Gazette.
For Thai children with dual nationality, the Citizenship Act (3rd issue) 1992 states that a Thai national with a foreign father may declare his or her intention to revoke Thai nationality within the year following the child’s 20th birthday.
The law does not mention that the child will automatically have his or her Thai citizenship revoked. If the child does not declare his or her intention to revoke Thai citizenship, that child will still hold Thai nationality.” Monday, April 12, 2004 6

26 PM
Satchaphand Atthakor, Deputy Director-General, Consular Affairs Department, Ministry of Foreign Affairs.
source:
Issues & Answers
That is the Thai side of the equation.